In Monodevelop ; Go Project > Assembly CSharp Options > Build > General , and check if the correct Framework is setup.
When i change it from 3.5 to 4.5.2 or 4.6.1 it worked out for me, but sometimes when i restarted the Monodevelop it again automaticly changed back to 3.5
I had this same issue. Every suggestion did not resolve it, even rolling back earlier commits, the problem persisted. Deleted all sln files, no change. The only thing that worked is using Visual Studio instead of monodevelop.