Please check -> If your payment method is declined in the App Store or iTunes Store, see ->
If your payment method is declined in the App Store or iTunes Store - Apple Support
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How to complete your purchase if a payment failed or was declined
- On your iPhone or iPad, open the Settings app.
- Tap your name.
- Tap Payment & Shipping.
- Add a different payment method and remove the old one.
- Try to complete your purchase.
If you still can't make a 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.
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.
If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)