How i can buy in app purshases with apple gift card

See Change, add, or remove Apple ID payment methods https://support.apple.com/en-us/HT201266.

Also, see Add funds to your Apple ID https://support.apple.com/en-us/HT208687

"Before you can add funds to your Apple ID, you must have a valid payment method on file. Learn about the payment methods that you can use with your Apple ID. ... See how to Add funds to your Apple ID https://support.apple.com/en-us/HT208687  Note that "after you authorize the payment, the funds ... [are credited] to your Apple ID. Then, you can use the credit to make purchases from the App Store or iTunes store. Need more help? Contact Apple support for help here: Get started."

Redeem the Gift card to your account and you should be able to use that to make the purchase.

Gift cards redeem.    

Gift Card - can’t redeem. 

Do you have apple gift card or iTunes/app store gift card?