why I can not use the balance to buy things in the game

See -> Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

-> set your Apple ID balance as you defaulr payment method.

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

Click here and ask the iTunes Store staff for assistance. Only they will know; this isn't a problem with the payment method itself.

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