If you made a video using the QuickTime Player on Mac, you would check that it is not possible to export the video to mp4. More info.
And now, what to do? How to convert the video to mp4? To do so, you can use ffmpeg. Following the steps.
brew install ffmpeg
Now, to convert:
ffmpeg -i <your-file.mov> -q:v 0 <my-new-file.mp4> -vcodec libx264