Sometimes when this happens, it means you have exceeded the file storage quota limit. It may be good to see if you have enough storage space available for the file you are trying to add, especially if it is an ungraded discussion board. How do I view my user files as a student?
If your school has an integrated video management platform (Canvas Studio, Kaltura, Panopto, etc.) then you may want to try uploading your video to that and embedding it in your discussion board using that tool instead of directly submitting the mp4 video file.
