Unable to purchase game credits on the game app

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.

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

Contact Apple for support and service – Apple Support

Did you contact iTunes support for help as instructed?