If you don't have a credit card or don't want to use a credit card, you can also use your Apple ID balance as payment method.
See -> How to redeem your Apple Gift Card or App Store & iTunes gift card
How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)
When redeeming a gift card, the amount will be credited to your (1) Apple ID balance and (2) you need to set your Apple ID balance as your default payment.
(1)
See -> Check your Apple ID balance
Check your Apple ID balance – Apple Support (UK)
(2)
See -> Add a payment method to your Apple ID
Add a payment method to your Apple ID – 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.