I just noticed that spacemacs installed an old version of org-drill.

org-drill is out-of-date on elpa.  From my ~/.emacs.d/elpa/org-plus-contrib-20170717/org-drill.el :

    ;;; Version: 2.4.7

The official org-mode repository is out-of-date (2015):

http://orgmode.org/cgit.cgi/org-mode.git/tree/contrib/lisp/org-drill.el

    ;;; Version: 2.4.7

The new version is on BitBucket:

https://bitbucket.org/eeeickythump/org-drill/raw/01b05cd7561ad69e5ec9c1200414d4fa128c9a17/org-drill.el

    ;;; Version: 2.6.1