Your Apple ID payment method was declined

Based on your country of residence and country you have selected under your Apple ID --> Payment methods that you can use with your Apple ID - Apple ...

Have you a Debit/Credit card that is issued to you that is valid internationally and is from the same Country / Region as your Apple ID.

Know it here --> If your payment method is declined in the App Store or iTunes ...

Check the following

1. The payment method that you are adding is of the same country as that of App Store Country on your iPhone
2. You have a valid payment plan debit/credit card from an authorised bank from the same country
3. Payment methods that you can use with your Apple ID - Apple Support
4. Learn more about family purchases and payments.
5. Please check if your Debit/Credit card is valid internationally

Read this article carefully:--> If your payment method is declined in the App Store or iTunes Store

If you get no help, I suggest that you contact iTunes Store Support Team directly, using this specific link:--> Apple Support

(if you get no help, look at the bottom of the above link, where you can contact them using "chat" or "phone" options).

Regards

Giulio