Just a note of my findings.. In my general work to get this package building and running again, this is because the "python3 -m fava.cli" command fails to load/run for some reason.
In my case, it was a missing dependency, python3-watchfiles, since I was buiding on bookworm, but it could be anything that interrupts the python load. Run: PYTHONPATH="/path/to/fava/src" python3 -m fava.cli --help to try to duplicate and find the ultimate cause. - Chris