I'm having the same exact issue. I've followed the monogame setup instructions for mac carefully, uninstalled everything, ran through it again, restarted, etc etc.
Trying to figure out why it would essentially be saying "hey I don't have the .net assembly" when I thought that's like... what mono is... as in it essentially represents an assembly reference which replaces the set of assemblies that would otherwise be required, including .net 4.5. But my understanding here may be limited! I tried installing mono 5.16 (I had used 5.18 cuz it said that was what was best for visual studio) but that didn't seem to help either here.