probably 4.6.2.7+dfsg-1ubuntu1 from 2016. The LTS goal is not on staying current.
AFAIK the Duplicati autoupdater is not aware of these things, and also can’t ask the user to look.
There are other problems with it, and someday updates might be done using a different method.
Meanwhile, the release notes cover known mono needs.
Release: 2.0.5.1 (beta) 2020-01-18 (at least mono version 5.0)
Release: 2.0.6.1 (beta) 2021-05-03 (at least mono version 5.10)
The issue here, though, seems like just an old bug, and version requirements are not bug-based.