Compressor Kit errors can be frustrating.
Take a look at this thread and see if anything in there helps you:
https://discussions.apple.com/thread/8421678
Also, sometimes a reinstall of iMovie is what is needed to solve the issue.
Drag your present iMovie app from the Applications folder into the trash, but
do not empty the trash. Then sign in to the app store and open your purchases
folder by clicking on your name at the bottom of the app store side bar. You
will see a display of your purchases. Redownload iMovie from there. If
unsuccessful, drag your present iMovie out of the trash and replace it in the
Applications folder.
-- Rich