Why can’t I order Apple gift card on my iTunes Store?

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

By the way, Apple is bad about naming things and creates a lot of confusing terminology. An "Apple Card" is a credit card service offered through Apple. What you want is an Apple Gift Card. When you "order an Apple Card" that is normally interpreted as meaning you want to get a physical version of the credit card which is not what you are doing.