Hey @Moliks, have you tried these troubleshooting steps for the Stadia App already? Sometimes one of them can fix the issue.
Also check if your Google Play Store is updated, see more info here. Personally I would also recommend to check if the Google Services App is updated too.
After all steps: Does it work or still not? Is there any error message appearing?
If the problem does still exist I would recommend to contact Stadia support to figure out if this might be a known issue with your device or if they have further steps.
Yeah could be that it's buggy with a specific device or Android version, I have no idea. Might be worth to also leave feedback for that in the Stadia app.
If you have any chance, try to ask a friend of yours who has Stadia, to login with your account to the Stadia app and purchase games from there. Should be a friend though who you can trust and don't give your password away, just borrow the phone if possible.
Same issue here. When I press buy button sometimes nothing happens, sometimes I see "Sorry something wrong on our side, Please try again later". Can not buy any game.