Your purchase could not be completed
Why i m facing the same issue with different cards
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Various forms of this old and somewhat unspecific message ultimately lead 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 there's a problem. Check with your financial institution about the status of your card. Also make sure that the name and address with Apple correspond exactly to that which your bank has for you.
If nothing in that document provides resolution then Contact iTunes Support at: https://support.apple.com/choose-country-region/media-services