Know it here --> If your payment method is declined in the App Store or iTunes ...
Check the following
- The payment method that you are adding is of the same country as that of App Store Country on your iPhone
- You have valid payment plan / debit / credit card from an authorised bank from the same country
- Payment methods that you can use with your Apple ID - Apple Support
- Learn more about family purchases and payments.
- Please check if your Debit/Credit card is valid internationally