Did you try adding it again? If that is not working, can you provide more information such as error messages? I believe it is possible that if the payment method failed with a previous purchase that it could be flagged as a problem method, in which case you will have to contact Apple about entering it again.
See this article about payment methods that you can use with your Apple Account --> https://support.apple.com/HT202631 then see this article about how to add a payment method to your Apple Account --> https://support.apple.com/HT201266
This support article has various country telephone numbers for contacting Apple for support and service ➔ https://support.apple.com/106932 Scroll down to “Connect with live support”. If your country is not in the list, go to the “Other countries” entry at the end of the list and click on the “options” link. Not every country has telephone support, and phone numbers may only work when calling from the same country.