New payment method
Each attempt to add a new card it says invalid security code
iPhone XS Max, iOS 12
Question marked as
Top-ranking reply
Here are the instructions for changing the payment method-->Change, add, or remove Apple ID payment methods - Apple Support
Note that at this time (9:28 AM PDT, May 8 2019) the iTunes store is having problems-->Apple - Support - System Status, so that may be the real issue.