MP4 files cannot be opened for writing

I thought I read that mp3tag works for editing tags on mp4 files. But, I'm getting an error message when adding a new thumbnail to an mp4 video, and it won't allow me to save it.

See MP3tag's main page for supported file formats.
https://www.mp3tag.de/

Which one?

mp3tag error message

Here's a screen shot of the message.

This error message could have several root causes:

  • you do not have sufficient access rights for these files (you should be the owner, being Admin is often not enough),
  • the file is blocked by another program so that it cannot be written,
  • the file is corrupt in some way.

Depending on what it is, the remedies vary.

It could be number two on your list. At some point, even though the error message indicates that the file couldn't be written on, the thumbnail changed to the picture I used in the tag. So, I'm wondering if the error resulted due to the file residing in my OneDrive folder, and OneDrive is constantly backing it up to the cloud? Just a thought.

see e.g. here:

Yah. That sounds like that's the issue. I'll move the file to another folder and see what happens.