Updating/Upgrading your iMovie version and operating system probably will cure the CompressorKIt error. It least, it has for others.
Try updating iMovie first. To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the trash. (You can skip that step if you had iMovie 10 on your computer before, but perhaps deleted it.) 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 your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.
If that doesn't work, you might try upgrading your Sierra operating system to Mojave, if your computer is not too old. Check out this link re the requirements for Mojave:
https://support.apple.com/en-us/HT210190
-- Rich