Open iMovie in a newly created library. To do that, hold down the Option key while opening iMiovie, and choose to create a new library from the dialogue box that appears. Then create a test project in the new library and see if you can share it out.
If still no luck, try the following procedures:
Opening iMovie in a new user account
Create a new user account from System
Preferences/Users and Groups and try to open and use iMovie in the new account.
You can use the Guest account on your Mac for this purpose.
Safe Mode
You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads,
immediately shut down and reboot in normal mode. Then see if iMovie works properly.
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, although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
Contact Apple Support
If none of the above resolve your issue, contact Apple Support. The techs there have the capability of remotely accessing your computer that we cannot do here. To contact apple support click on the Contact Support item in the upper right hand corner of this forum’s screen. Navigate to a place where you can enter your phone number.
-- Rich