The Compressor Kit Error message has been reported with increasing frequency
of late.
Lately users with the problem have been able to fix it by reinstalling/updating/upgrading
iMovie and, if necessary, the operating system to Catalina. However, you already are running
Catalina. That makes it even more puzzling as to why you are getting this error.
Sometimes successful is starting up in Safe Mode (see Finder menu for
instructions) and then after everything loads immediately shutting down and
rebooting in normal mode. However, you already have tried Safe Mode without
success.
Recently there has been a breakthrough on this issue, discovered by
nicocaro, who posted a solution on this forum. Below is a link to the thread
where he describes how to solve the problem by removing a 64bit codec from an
iTunes update. Other posters have successfully tried the solution. It
seems that updating iTunes may cause iMovie to exhibit the CompressorKit:Error
by implanting a 64bit architecture in the system library, that conflicts with
an older version of iMovie. That would explain why upgrading to 64bit Catalina
and current version of iMovie has solved it for some posters here. Upgrading
to Catalina, however, would disable all older 32bit applications that would be
no longer supported. Some legacy media may not play in iMovie, either, and
would need to be converted.
Here's the link to nicocaro's solution:
https://discussions.apple.com/thread/251266351
And here as well:
https://discussions.apple.com/thread/251352619
Here's another link, though older, that describes what others have tried with
the Compressor Kit Error:
https://discussions.apple.com/thread/8265338
There may be some proposed solutions in the above link that will work for you.
All else failing, try calling Apple Support. The techs there are very savvy and
also have the capability of remotely accessing your computer. Click on the Contact
Support item in the upper right hand corner of this screen and navigate to a
place where you can provide your telephone number so that Apple Support can
call you back. I think that the file path is Contact Support/Mac OS/Applications/iMovie.
Good luck with this.
If you have time, post back with your results, as it may assist others who have
this perplexing issue, and will contribute to our data base.
-- Rich