I saw this problem for other Mac users, apparently this is an error with a couple of applications beside Backrooms, try this:
1- Open terminal.
2- Enter sudo chmod -R 777
3- Drag and drop Backrooms into terminal.
4- Click enter
5- Enter your password
i can solve your problem :0. all you need to do is install the itch.io app and it'll give you access to any game. This solved my problems with opening several other games. I hope it helps. if you have any more questions, please feel free to ask. I'd certainly walk you through if you need any more help
