iMovie for Monterey - THE FIX!
this worked for me on Monterey 12.7.5
go into applications, drag iMovie to the trash and empty trash.
then go to app store and click on iMovie (ver. 10.4)
a window comes up that says it is not compatible, but that you can download an older version.
click download an older version, then let it install as usual.
iMac 27″, macOS 14.5