Why can't I download my iMovie projects?

Re: I can no longer download my Imovie projects

You may want to check if yoor version of iMovie is up-to-date with your MacOS.

Eg: Check for updates in the App Store

or

See how you can: Export a movie, trailer, or clip as a file in iMovie on Mac - Apple Support

More info: if needed: iMovie User Guide for Mac - Apple Support

I'm not sure I understand what you are doing. Where are your iMovie projects that require you download them?

Mine are all on an external drive. Did you upload them to a cloud service?