Hey @PeterPayan ,
Thank you for posting on the Instructure Community!
I'm sorry to hear that you're having trouble with your media converting for your assignment submission. When you go to your assignment and view the submission, does it give you an option to download it? It would be in the top right-hand corner where it says 'Submitted!'. I'd try downloading the file, and then running it through a software such as CloudConvert, and making the file a .mp4 file and then re-submitting it, if your instructor has resubmissions turned on. If not, I'd recommend reaching out to them and seeing if they have another way they'd like you to re-submit it another time.
I hope this helps you out, let us know if you have any additional questions or concerns!
All the best, good luck!
Noah