Hi,

I loaded the pandoc-mode:

M-x pandoc-mode

and there came up this messages:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
   string-match("markdown" nil)
.................................
Warning (comp): hydra.el:103:1: Warning: defvar 
`hydra-amaranth-warn-message' docstring wider than 80 characters Disable 
showing Disable logging
Warning (comp): hydra.el:238:1: Warning: Alias for 
‘hydra-hint-display-type’ should be declared before its referent Disable 
showing Disable logging
Warning (comp): pandoc-mode.el:1166:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1167:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1255:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1239:53: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1251:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1252:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1253:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1254:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1282:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1279:49: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1288:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1285:50: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1303:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1304:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1305:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1343:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1337:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1352:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1346:65: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1370:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1355:67: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1367:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1368:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1369:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1379:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1373:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1388:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1382:62: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1382:62: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1382:62: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1382:62: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1397:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1391:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1391:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1391:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1391:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1391:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1391:63: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1406:8: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1400:60: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1400:60: Warning: docstring wider than 80 
characters Disable showing Disable logging
Warning (comp): pandoc-mode.el:1167:8: Warning: the function 
‘pandoc-output-format-hydra/body’ is not known to be defined. Disable 
showing Disable logging
Warning (comp): pandoc-mode.el:1166:8: Warning: the function 
‘pandoc-input-format-hydra/body’ is not known to be defined. Disable 
showing Disable logging
....................................................................

I don’t know what to do
...............................................................
I have got this about "markdown" in my init.el file:

;; markdown for formatting text
(defun rcd-markdown (text &rest args)
   "Markdown processing"
   (let ((markdown (executable-find "markdown")))
     (cond (markdown
	   (cond (text
		  (apply 'rcd-command-output-from-input markdown (append (list text) 
args)))
		 (t ""))
	   (t (user-error "Command `markdown' not available"))))))

(defun rcd-markdown-preview ()
   "Preview Markdown."
   (interactive)
   (let* ((output (rcd-markdown (buffer-string)))
	 (file (concat (buffer-file-name) "-MD.html")))
     (with-temp-file file (insert output))
     (browse-url file)))
...........................................................

Kind regards

Gottfried