I also had this issue on a fresh bullseye install. I was able to get a different error message by commenting out the reference to /etc/bumblebee/xorg.conf.d in /etc/bumblebee.conf, but then I get a different error about a missing module 'mouse'.
Unfortunately I didn't have any time to troubleshoot any further and I think there are other issues at play outside the scope of this bug (like the fact that with the legacy Nvidia drivers there is no 'nvidia' module so it's trying to use nouveau) so I'll probably have to try to solve all of that before I can offer any more useful feedback on this bug.
