I concur with this comment (comment that was never replied to)
following these instructions (meant for Ubuntu 20.04):
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb 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-complete
one can run ‘mono’ without problem, and Duplicati can be installed.
I have a VM under Kubuntu 22.04 where I tested successfully the most recent stable release in graphical mode (the trayicon stuff).
Edit: all this is done of course on a clean installation. Attempting to upgrade the distro on a damaged installation with standard Ubuntu instructions will usually lead to disaster.