Install Mono
Offizelle Seite:
https://www.mono-project.com/download/stable/#download-lin-ubuntu
Add the Mono repository to your system
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
Mono
sudo apt install mono-complete
The package mono-complete should be installed to install everything – this should cover most cases of „assembly not found“ errors.
Mono unter Ubuntu installieren