A couple of other people have posted similar messages on this forum. I don't know what is causing you to receive the compound clips error message. While Final Cut Pro has a feature for creating compound clips, iMovie does not. A compound clip is like a little mini project within a project. It can be a combination of audio and video clips, and titles that are joined together and inserted or created in the main project.
You might try creating a new project and see if that cures it.
Also you can try deleting preferences by opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new blank library. Reopen your old library to get back to your projects.
You can try redownloading iMovie by moving your current iMovie app (not the library) from the Applications folder to the trash, but not emptying the trash. Then sign into the app store and view the apps listed in your Accounts Purchases. iMovie should be one of them. Update iMovie from there. If unsuccessful, drag your present iMovie app from the trash back to the Applications folder.
If none of the above works, you might benefit from contacting Apple Support and talking to the techs there about this issue. They have the ability to remotely access your computer that we do not have there. If you have time, post back and let us know what they said at Apple Support.
-- Rich