Why can’t I add an Apple Store gift card to my wallet?

This article explains the different kinds of cards Apple uses and has links on how to redeem them and what can be purchased with each kind. Click on the link --> What type of gift card do I have? - https://support.apple.com/HT204199

'If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card' click here --> http://support.apple.com/HT201195

What happens when you try to add it?