If there is sufficient disk space, this is not a space issue, try Fix 2.
25GB is recommended, and the more, the better.
Here is how to check the available storage and the file size of iMovie Library. So first go to ensure this is really a disk space related problem when you see the error message prompted as above. The iMovie library file could be tremendous if you go to check its size. The iMovie Not Enough Disk Space issue might not happen on new macOS desktops or MacBooks, but it does appear on many machines if they have been using iMovie frequently, when you try to import media to iMovie for editing. Problem Fixed: iMovie Not Enough Disk Space Error