I was trying to buy iTunes gift card with my iCloud and they said I should contact iTunes support

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