Sorry for late reply.
In my sdk folder there are two folders: 3.1.406 and 5.0.200.
I think you need to have only one .net version installed. 5.0. for example.
Then I followed the instructions https://community.monogame.net/t/install-monogame-3-7-1-on-linux-mint-19-2-tina-cinnamon/11793
I think you will have to change this line: "
echo "deb https://download.mono-project.com/repo/ubuntu vs-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-vs.list "
according to your ubuntu repo.
In my case “ubuntu vs-bionic”. I got monodevelop and monodevelop-stable. Stable doesn’t work.
Then you run monodevelop there should be monogame project.
And I changed “any CPU” onto “anyCPU” in the .sln file. Project- RMB - tools - edit file, to avoid red cross on the project.