My app store is set on the U.S. and I tried to purchase something, i entered my master card number with the code and everything and it pops up an error saying that it is declined
Why is this happening
iPhone 6s, iOS 12
Assuming that your store has always been the U.S. App Store, look here for reasons why the card could have been declined.
If your payment method is declined in the App Store or iTunes Store - Apple Support