I found the problem.
Recently I changed the python default from 2.7 to 3.7 using [update-alternatives].
I installed python3-docutils (Debian Buster) and now make completes successfully.
Though I wonder if the Geany configuration files need a little tweaking to find the problem before `make` is run. I don't know a better method of checking (I'm not at all familiar with python packages).