"We could not complete your purchase" error occurs when purchasing using Apple Gift Card

I charged $25 with an Apple gift card and tried to make a purchase through the app.

When I entered my password, I got an error message saying "We could not complete your purchase"" and the payment wouldn't go through.

I can confirm that my account balance is $25.

What should I do?

[Re-Titled by Moderator]

Original Title: We could not complete your purchase