Scripts not attaching when opening in 2021.3

[tacman]

When I fork and open the final project in 2021.3, and open the demo scene and run it, I get errors because the scripts don't seem to be available. This, for exanmple, is in the Player inspector.

I suspect that this has nothing in particular to do with this repo, but rather with migrating to the new LTS version of Unity, but nonetheless perhaps I'm doing something wrong when opening it.