Unfortunately, this looks like an issue for the MonoDevelop forum however, if someone can assist here, hopefully they will do so over the coming days, however, I advise you to head over to the MonoDevelop forum and ask there too.
Hey @Kasia , I have also been using MonoGame on macOS for a few months now, and have run into the same issue time and time again. I do not have a fix, but I do have a workaround (it’s tedious and annoying, but it’s been working for me so far).
After you initially install the extension, you can ignore the error, and then try to create a new project. If you see the MonoGame templates, then it is working, regardless of the errors. I have never seen this not work on a fresh install of the extension.
Once you get the extension to be active, it should remain active until you terminate VS. The next time you launch VS, the extension will not be working – to re-enable it, go into your extension settings, uninstall it, and re-install it again. It’s very important that you actually uninstall it and then re-install, if you skip the uninstall and just replace the existing, installed extension, it will not work. Repeat every time you start up VS.
I keep a copy of the extension right on my computer, so re-installing it just takes a few clicks. Tedious, yeah, but it works.
EDIT: Oh, and by the way, you really only need the extension when creating new projects. The extension doesn’t really do anything you need while working on your existing projects, it just makes the MGCB file launch in the MCGB editor program instead of as as text file in VS. 99% of the time, I work on my game with the extension disabled, and I don’t notice any problems.