I’m having a similar problem, but it may not be related.
I’m on ubuntu 16.04.
Installed monodevelop 6.1.4 via flatpak.
Installed monogame 3.5 from monogame.org.
Addins working (18.104.22.1686), project created.
Getting error: TestProject.csproj could not import “$(MSBuildExtensionsPath)\MonoGame\v3.0\MonoGame.Content.Builder.targets” (TestProject)
I’ve been using monogame + vs for years on windows but finally made the switch to linux, so this is probably a newbie mistake. If you could explain the problem like I’m 5 years old, that would be great. Apologies if my problem is separate from theirs. Thanks!