Run the following commands to install mono
sudo apt update sudo apt install dirmngr gnupg apt-transport-https ca-certificates software-properties-common sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF sudo apt-add-repository 'deb https://download.mono-project.com/repo/ubuntu stable-focal main' sudo apt install mono-complete
Progress will look like this
Confirm if the package is installed by running the following command:
apt -qq list mono-complete
The output should look like this:
mono-complete/stable-focal,now 6.12.0.122-0xamarin1+ubuntu2004b1 amd64 [installed]