The CompressorKit error can usually be fixed by updating your software. Your operating system of 10.12.6 is fairly outdated. You didn't mention what iMovie 10 version that you were using. If you can upgrade your operating system to Mojave and your iMovie app to the highest version that Mojave will run -- it should be iMovie 10.1.14 -- that might solve it.
This link will show you how to upgrade to Mojave:
https://support.apple.com/en-us/HT210190
If your computer is too old to enable you to upgrade to Mojave, then upgrade your iMovie app to highest version that your present operating system can run. 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 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.
-- Rich