can't access photos. from imovie

The very latest iMovie update I believe addressed that issue, so you might try updating if you haven’t already.

If the above doesn’t address your situation, check your Photos app General Preferences and make sure that the box that days “use as Photos system library” is grayed out, to insure that the library you are using is the system library. iMovie will only read the system library.

If you are using optimize storage with iCloud Photos iMovie won’t read those photos either.

— Rich