i am unable to add a payment

Are you talking about a payment for Apple ID or for Apple Pay?

For Apple ID go here - Change, add, or remove Apple ID payment methods - Apple Support

For Apple Pay -

• If you are getting a message to contact the card issuer, call the bank or credit card company who issued you the card to make sure there are no restrictions on the card. 
• If you are getting a message that Apple Pay services are unavailable at this time, sign out of your Apple ID and back in.
• Check your Apple ID region to make sure it is correct (https://appleid.apple.com/)
• Check the region on your device to ensure it is correct (Settings>general>language & region - set to your region)
• A passcode is required to use the Wallet app. 
• You need to be in a region that supports Apple Pay - Countries and regions that support Apple Pay - Apple Support
• Your bank needs to support Apple Pay - Apple Pay participating banks - Apple Support
• Sometimes if you sign out of your Apple ID and back in it resolves the problem.
• If none of these suggestions are able to resolve your issue you can go here for further assistance - If you can't add cards to Wallet for Apple Pay - Apple Support