I'm sorry you're having trouble with that! Are you downloading directly from Itch or through the App? And does your phone have any firewall/virus scanner programs? Those sometimes decide the file is weird and won't let it be put on the phone so turning that off could help. It's also possible that Itch is just being weird and sometimes you can't properly download things and then the issue just goes away the next day or so.
Were you able to play the game in the past? If so, maybe try removing any old files you have of it on the device in case your phone thinks you already have the game and that's why it can't be readded. I hope one of those ideas helps!