I have a MacBook, I bought it for work, and I thought if I bought it I could work without interference. But for some reason, the built-in QuickTime Player app on MacBooks can't always open MP4 videos. I don't understand why this happens, and it's important to me for my work.

Often MacBook owners complain about the problem that Quick Time Player cannot open videos, this is due to the difference in encoding format. I don't really understand this. Maybe you should read this article: https://mac.eltima.com/mp4-player.html . I think it says how to solve this problem, but you may have to use a different video player. With the new video player, I think you won't have any more problems with the video.

If you bought a MacBook, it doesn't mean you won't have problems with it. Yes, there will be fewer problems than with ordinary laptops, but the MacBook is not perfect.