Unable to purchase game credits on the game app

In this case I'll advice you to contact Apple Support.

Contact Apple for support and service – Apple Support

I'll advise you to check your payment method, in some cases there might be an issue with card.

Also you can check your restrictions in Settings > Screen Time > Content & Privacy.

If issue still persists try to sign out from Apple Media Services account, restart device and sign in again.

Did you contact iTunes support for help as instructed?