Do you mean a "Your purchase could not be completed" message"? That is not what you said and it helps to be accurate when posting to this forum.
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