The 1s Movie editing feature is only available to Album Admins of albums subscribed to Premium Pro. This feature only supports one person making changes at a time. This means that if an Album Admin is editing a 1s Movie and another Album Admin tries to edit the same 1s Movie, the second Album Admin will get an error message that says "Another Album Admin is editing this 1s Movie".
If you receive this message, please wait for the other Album Admin to finish before making your edits.