Run mono --version and if it isn’t 5 or greater, install one from here. I usually favor mono-complete.
EDIT:
The 2.0.5.1 mono requirement went up, so after mono upgrade, you might as well put 2.0.5.1 back, or possibly you’ll find that it’s still there. A Duplicati update goes into a different area, so downgrades are somewhat tricky. The database format can get you too, however there’s a backup of the old DB made.