my master card doesn’t work. why not?
why won’t my mastercard work?
iPhone XS
Work for what? I use one of mine for scraping the car window in winter.
Have you asked your card issuer?
If your payment method is declined in the App Store or iTunes Store [or you have a billing issue with a previous purchase in the iTunes Store] - https://support.apple.com/HT203005
If you can't remove your last payment method or use no payment method with your Apple ID - "Learn why you can't use your Apple ID without a payment method in the App Store, iTunes Store, and more." - https://support.apple.com/HT203905
#1 in the last link is a frequent cause, where somebody has a subscription which means you cannot change the payment method. You must wait for the subscriptions to expire, or contact Apple and see if they can resolve the issue.
This is the most direct method to contact Apple for media/app/iCloud store support:
iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions -->
https://www.apple.com/emea/support/itunes/contact.html
Use this method to get telephone support:
Contact Apple for support and service - "See a list of Apple phone numbers around the world."
https://support.apple.com/HT201232