Welcome to the forum @John_Davis
Does that install dependencies? I thought that was a dpkg omission. The apt command can, however Duplicati actually recommends installing mono directly from that project per manual, however since manual stops at Ubuntu 20.04 (needs update) you can get some advice here.
It looks like your distro might have (via Ubuntu) mono 6.8.0.105 which is 2020, so a little old, although changes are few these days. Duplicati is in the process of ending its usage of mono.
An alternative to apt is probably to install from your file browser which might install by gdebi.
Duplicati 2.08.1-1 search gtk-sharp2… might be the next thing that the new LM 22 is missing, possibly due to a move to GTK 3. II don’t know if libgtk-sharp3 could easily work in its place.
Regardless, if 2.0.8.1 fails there, there are test releases such as 2.0.9.104 Canary and others