I don’t know what mono would have to do with it, but the work after the early reports found below:
VSS warning on non-Windows platforms #4149
AlphaVSS.Common is only used on Windows and has long been removed on Linux fresh installs.
Problem is that a bug fix in a different area caused non-Windows systems to try to load it anyway.
Do not remove Alpha VSS dependencies in installer packages #4178 is one proposed fix method.
Users who fresh install an earlier Duplicati then have Duplicati autoupdate might not see this bug, because the updates contain all the files. Until a fix gets into Beta (which can take awhile), maybe
https://github.com/duplicati/duplicati/releases has a .zip file for your version. You can grab that .dll.
or kind of ugly in a different way, but also cleaner in a way:
2.0.5.0_experimental_2020-01-03 can probably be fresh installed, change Update channel to Beta, allow Duplicati to install 2.0.5.1 as an update, and the update will probably contain a full set of files.