So this can be a silly question and i might be overlooking something but i am not able to see an option to upload a mp4 video. I can see videos being shared in posts but i am nit able find that option.
Is it restricted based on your level?
My Apologies again 
It’s most likely not a direct upload like we can do with pictures, but most people either use a YouTube or Loom link, or some people create a gif image, although I’m not a big fan if gifs because there is no media controls on them.
Basically, if you can host the video somewhere else and post the link to it, that usually works.