Your problem is the set-version you have is setting an empty string as the version so snapcraft is assuming that it failed. Your tar.gz file includes the JabRef.cfg that you’re attempting to parse, but that file does not include a line with the text app.version= which is the text you’re trying to match with grep so the whole command returns an empty string.
I tried changing the script to snapcraftctl set-version "$(git describe --tags | cut -c2-)"
but I got the same error on a remote-build
(I cannot use the standard git version because it selects the incorrect value…)