The CompressorKit error code has come up quite frequently on this forum. It is a stubborn issue. Apparently it is caused by an iTunes update that implants 64bit architecture in the library. However, this would be an update to the Mac. I don't know if the same issue happens with iTunes updates on an iOS device.
I have heard, but not personally confirmed, that Apple has fixed the problem with a supplemental update to the Mac OS.
So you might want to check with the online app store.
If the above is not the case, then the status of this issue is as follows: No universal fix, although lately users with the problem have been able to fix it by reinstalling/updating/upgrading iMovie and, if necessary, the operating system
to Catalina. Also 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. 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 architecture 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
