sudo apt install gnupg ca-certificates
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb [arch=amd64] https://download.mono-project.com/repo/ubuntu stable-focal main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
sudo apt update

sudo apt install mono-devel

Notes:

The package mono-devel should be installed to compile code.

The package mono-complete should be installed to install everything - this should cover most cases of "assembly not found" errors.

The package mono-dbg should be installed to get debugging symbols for framework libraries - allowing you to get line numbers in stack traces.

The package referenceassemblies-pcl should be installed for PCL compilation support - this will resolve most cases of "Framework not installed: .NETPortable" errors during software compilation.

The package ca-certificates-mono should be installed to get SSL certificates for HTTPS connections. Install this package if you run into trouble making HTTPS connections.

The package mono-xsp4 should be installed for running ASP.NET applications.

https://www.mono-project.com/download/stable/