The Compressor Kit Error message has been reported with increasing frequency
of late. No universal fix so far, although lately users with the problem have
been able to fix it by updating/upgrading iMovie and, if necessary, the
operating system. 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.
Here's a 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,
or something like that.
Good luck with this. If you have time, post back with your results, as it
may assist others who have this perplexing issue.
-- Rich