how do i get app in app purchases ?

You may need to have a valid payment method on file.

See the following Apple articles:

Buy additional app features with in-app purchases and subscriptions

Buy additional app features with in-app purchases and subscriptions - Apple Support

If you can't find or use your in-app purchases

If you can't find or use your in-app purchases - Apple Support

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See -> What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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Apple Gift Card and Apple Account balance can't be used for some purchases

• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

See also -> Payment methods you can use with your Apple ID

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

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

Add a payment method to your Apple ID -> Add a payment method to your Apple ID – Apple Support (UK)

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

If you update your payment method or add funds to your Apple Account 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.