I've never used MS Paint seriously but now I've been playing around with it for a while and it's actually kinda charming! I'm really looking forward to this jam.
Yeah, it's not really meant for game making or anything like that. The main problem is that, as far as I've seen, there isn't a layer tool. That can make a lot of complications down the line if you want to animate something.
The problem isn't your itch.io account, it's that you didn't upload the build properly. The error message is very helpful; you didn't include UnityPlayer.dll, and you probably also missed other necessary files.
