TL;DR: I, too, have been trying to get MonoGame to work on my Mac with no success. (Mojave 10.14.6)
First I tried following the official installation instructions, which as has been pointed out lead to installing incompatible versions of MonoGame and Visual Studio.
Then I tried downgrading to Visual Studio 2017, and was finally able to create a project (since the project templates finally showed up). However, I couldn’t get any project to compile, because of the same error as this person, which is apparently because Visual Studio 2019 (so I assume 2017 too) does not support Mono 6.0.
So I downgraded to Mono 184.108.40.206, and now get
Error MM0001: This version of Xamarin.Mac requires Mono 220.127.116.11 (the current Mono version is 18.104.22.168).
Conclusion: unless at least one of my sources is incorrect, there exists no compatible combination of MonoGame + Visual Studio for Mac + Mono that is currently available.
Am I missing something, or would that be correct? I would love to use MonoGame, but I have already spent a couple days trying to install it unsuccessfully, and would much rather believe it’s just me being stupid than assume that it is actually currently impossible to install.
Any help is much appreciated!