Apple Pay won’t let me add my mastercardgiftcard

I got a Mastercard gift card sent to me at my work. The gift card is by Mastercard themselves and even has an add to Apple wallet button on the website. Whenever I try to add my gift card Apple Pay says “unable to add card contact the card issuer”. But Mastercard itself has literally no support or customer service for this. Does anyone know why this is happening to me. The gift card was sent to me via email literally from Mastercardgiftcard.com. It works on websites as I just purchased something but I really want to add it to my Apple Pay. Does Apple pay just not support this gift card?