I can’t by in app purchases on my games

If you want to make purchases or update / download apps, including free apps, you need to -> update your payment method or add a new one. 

Then you need to set your Apple ID balance or a credit card as your default payment.

See also the following Apple articles:

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

Add money to your Apple ID balance

Add money to your Apple Account balance – Apple Support (UK)

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 for more support and assistance.

See also -> Related Article -> If your payment method is declined in the App Store or iTunes Store

• If you can't make purchases or update your apps, including free apps, there might be a problem with your payment method. To fix this, add a new payment method or update your payment information.