hi i also have that error when i was publishing my game too. i it happen for 2 reason:
internet connection
game sizes
if your internet is good they u need to decrese your game size i did by going to (project content and there was a small folder called imported content) and i deleted the build i didn't needed and got my game size down. If there is dialog box saying that the item are unused delete those item as they are takking space in your game. if item are being used it will say that they are being used and click on (NO FOR ALL) and it will delete the item that are in that build but are not in use.by doing this you can get your game size down it might help you to upload your game. Even after doing this it will take a few tries as it took me like 6 to 7 tries to publish my game,