Why can’t I apply my credit card

Probably you have the wrong Country / Billing / Shipping Address in the Apple ID. This should be the same as the issuing Bank Country.

Log in to https://appleid.apple.com with your Apple ID credentials.

Scroll Down to the Payment and Shipping Section

Tap on Edit

You will be given both Billing and Shipping Addresses to edit.

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