why is my payment method decline

I see you marked your post as Best Answer. People normally mark a post as Best Answer when it answers the question and no other replies are needed. If you mark a post as Best Answer when you are still looking for an answer, it may slow down any responses to your question. It looks like you are still looking for an answer.

There are many different reasons why your payment method may be declined. Is there an error message when you try? If yes, what does it say? Here is some advice from Apple. I've copied the pertinent part.

If your payment method has been declined in the App Store or iTunes Store - Apple Support

• If you want to know why a debit card, credit card or other payment method has been declined, speak to your financial institution. Apple doesn't have information about why the payment method was declined.