"There is not enough disk space available at the selected destination" in iMovie but I have over 300 GB free

The insufficient disk space issue can be solved by redownloading iMovie.

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the

trash. 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 and return it to the Applications folder. However, it should be successful and that will cure the not enough disk space problem.

-- Rich

Yes, you may now empty the trash of your old iMovie app.

-- Rich