iTunes purchase
Your purchase could not be completed.
iPhone XR, iOS 14
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Various forms of this old and somewhat unspecific message ultimately lead you to: https://support.apple.com/billing
The most logical selection is then "Can't complete a purchase?" which brings you to this document --> 'If your payment method is declined in the App Store or iTunes Store'
Click here --> https://support.apple.com/HT203005 It mostly just suggests you make sure your payment method is up to date or to try a different one. Apple is possibly just checking with your financial institution if the payment method is valid and getting a reply that it isn't. Check with your financial institution about the status of your card.
If nothing in that document provides resolution then Contact iTunes Support at: https://support.apple.com/choose-country-region/media-services