I have found a bug in which when you go to buy cards it does not let you leave that part unless you press the back button in Renpy. This happens on PC. I don't know if the same thing happens on Android (this is translated into English xd)
Is this at the start of the game? Did you buy enough cards? If that's the case, we've made it much clearer in later versions that you need to buy multiple packs of the first booster packs before you're allowed to exit.
Just keep buying cards until the exit button appears, that means you've bought enough. We do that so the player gets free cards to start the game and doesn't accidentally spend their own starter money on the cards
