Movies saved to iCloud Drive might display an alert message when you try to use them in iMovie for Mac:
"The document could not be opened. iMovie cannot open files in the QuickTime movie format."
To open or edit your movie in iMovie, move it to a locally connected drive (like your startup disk).
- Click the Finder icon in the Dock.
- Choose Go > iCloud Drive.
- Locate the movie you want to edit in iMovie.
- Drag the movie file to your desktop (or another folder that's on a drive connected to your Mac). If you want to work with a duplicate copy of your movie while leaving the original file in iCloud Drive, hold down the Option key while dragging to create a copy of the file.
- Open iMovie and import the movie file from your desktop.