Not sure what is causing that. Possibly it is related to the OS upgrade.
Try these procedures and see one of them resolves the problem:
Deleting Preferences
Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears.
iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
Resetting the System Management Controller
Try resetting the System Management Controller (but no longer available for the new M1 computers):
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM:
https://support.apple.com/en-us/HT204063
-- Rich