payment method

If you are talking about Apple Pay then follow these steps -

• How to use Wallet on your iPhone, iPod touch, and Apple Watch - Apple Support,
• Manage the cards that you use with Apple Pay - Apple Support
• If you have trouble adding a card to wallet;

1. Sometimes if you sign out of your Apple ID and back in it resolves the problem.
2. Check your Apple ID region to make sure it is correct (Change your Apple ID country or region - Apple Support)
3. Check the region on your device to ensure it is correct (Settings>general>language & region - set to your region)
4. A passcode is required to use the Wallet app. 
5. You need to be in a region that supports Apple Pay - Countries and regions that support Apple Pay - Apple Support
6. Your bank needs to support Apple Pay - Apple Pay participating banks - Apple Support
7. 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

If you are talking about Apple ID payment then go here - Change, add, or remove Apple ID payment methods - Apple Support