That error likely means either Coqtail wasn't installed correctly, it didn't load because of the file type, or it doesn't support your version of Vim/NeoVim.
To check for the first case, see if Coqtail is in your Vim runtime path (
:echo &rtp). If not, double check that you didn't miss a step while installing with whatever package manager you're using (e.g., forgetting to do
:PlugInstall with vim-plug). See https://github.com/whonore/Coqtail#installation-and-requirements for some options.
If it is installed, make sure the buffer you're editing has
filetype=coq. This should be set automatically if you open a
.v file, but you can also set it manually with
Finally, if everything is installed and you're editing a Coq file, check if
0. Coqtail requires Python 3.6 or newer and it will fail to load and print an error if your Vim/NeoVim installation doesn't have it.