The Compressor Kit Error message has been reported with increasing frequency
of late. No universal fix so far. The fix that has worked the most has been
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 pretty much describes where we are today
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 box that will ask for 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 somthing 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