MP4 cannot be opened for writing

I downloaded a movie from Microsoft Store.
They allow you to download a full copy as well as access a digital online copy.
I have been using MP3TAG and have had no problem adding covers to my own dvds.
When I try to add a cover to this dvd it comes up with Error message: cannot be opened for writing.
I cant add a cover no matter what I do. :frowning:

see e.g. here: /t/14151/1
sometimes MP4s have to get re-encoded to work.