I have moved my photos in external drive and when I try to open the photo or video it says "photos cannot play this video vecause the resource is unavailable" what can I do?

Hi

What format is the external drive. It needs to be APFS or MacOS extended (Journaled). If you've not formatted it it is probably ExFAT - which is NOT compatible with a photos library. You also need to be able to check the "ignore ownership" flag. (after unlocking the padlock.

Presumably you also backed up your library before moving it?