It could be that you have another version of mkdocs globally installed. That was my case.
I have recently begun using nala, a wrapper for apt, the package manager for the Debian family of Linux (I'm using Pop!_OS, a variant of Ubuntu). And it has a requirement for mkdocs from the distribution's repos, which I just realized.
When I typed mkdocs in the terminal, the wrong copy would be launched - the one in /usr/bin (actually in /usr/lib/python3/dist-packages/ according to mkdocs --version) instead of the one in my virtual environment. It's curious, because the virtual environment's bin folder was supposed to have priority in the PATH.
I uninstalled nala, which removed the globally installed mkdocs, and of course that fixed the problem, but I really like nala. I reinstalled it, and after reinstallation, the problem did not return. (Which does not make much sense to me.)
Next time, I will try to deactivate my virtual environment, then reactivate it to see if it fixes the problem. Or else I could check the path for the locally installed mkdocs (from the virtual environment) with which -a mkdocs and use its full path.