I'm working on the Opensimulator project. After upgrading Ubuntu 18.04 LTS to Ubuntu 20.04 LTS I was no longer able to compile the project using msbuild. msbuild could not be found. Trying to revert back and simply reinstall Mono according to the instructions for Ubuntu 18.04 on the Mono Project Download site didn't resolve the problem.
Finally I found a solution. I completely removed Mono from my installation. Even there is a stable-focal repository I decided to use the stable-bionic repository for a temporary solution until things get fixed.
As such I started with the instructions for Ubuntu 18.04 on the Mono Project Download site.
sudo apt install gnupg ca-certificates
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb https://download.mono-project.com/repo/ubuntu stable-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
sudo apt update
At this point I did no longer follow the instructions but installed the following packages:
sudo apt install mono-roslyn mono-complete mono-dbg msbuild
I could then use msbuild to compile opensim like before without any problems.
The Mono version is 6.8.0.105 which currently (2020/04/25) is the latest stable version anyway.
Note: it looks like both, mono-roslyn and msbuild must be separately installed. The same procedure described above does NOT work using the stable-focal repository.