David Hedlund wrote:

(defun switch-full-screen ()
  (interactive)
  (shell-command "wmctrl -r :ACTIVE: -b toggle,fullscreen"))

[...] let's focus why wmctrl cannot be executed from init in
Trisquel and Ubuntu MATE right now to avoid more confusion.

1. Make the terminal emulator window small so one can see that
   it isn't in fullscreen mode.

2. From the terminal, confirm that this works

   $ wmctrl -r :ACTIVE: -b toggle,fullscreen

3. If it does, in the Emacs window, do the same thing, first
   make it small so it is clear it isn't in fullscreen mode,
   then type and eval with C-x C-e or `eval-last-sexp'

     (shell-command "wmctrl -r :ACTIVE: -b toggle,fullscreen")

Thank you very much for helping out, Emanuel, and Björn.