Why can’t I use drm songs for my iMovie project

DRM-protected audio content can’t be used or embedded or otherwise copied into iMovie, per Apple:

Add music, sound effects, and voiceover in iMovie - Apple Support

...You’ll need to purchase sufficient rights from the music rights-holders or their licensing entity to use the music in your iMovie work.

Songs purchased from iTunes after year 2009 generally do not have DRM protection. So , if you need to use a specfic song you can purchase it from iTunes for about $1.29.

— Rich

Apple Music cannot be used in iMovie. It is a subscription service and its songs are DRM protected. Even downloading them into iTunes will not remove the DRM protection.

I believe that songs can still be purchased from iTunes, and they should be DRM free and usable in iMovie. You might try that route.

DRM (Digital Rights Management) means that the digital rights to use of the song are protected by law and are restricted by the owner of the rights.

— Rich