The "not enough space" issue has been reported by several others. The solution is to redownload iMovie. To do that, drag your iMovie icon from the Applications folder into the trash, but don't empty the trash. Then sign in to the online app store with your Apple I.D. and password. Open your app store account by clicking on your name at the bottom of the app store sidebar. Review your purchases list. iMovie should be there to download. Do the download and, if it is successful, you can empty the trash of your present iMovie app. If for some reason the download is not available, move your present iMovie out of the trash and replace it in the Applications folder.
-- Rich