Steam isn't accepting my debit card for some reason?

Since apparently you can't purchase EVE Online as a gift, my uncle is letting me use his account to buy it for him for Christmas. Unfortunately, Steam won't accept the purchase, even though there is nothing wrong with my debit card. It did this last time I tried to buy him a game using my debit card on his account, but I reported the problem to Steam Support and thought they would have fixed it by now (that was a while back).

Could it be because I generally only use my debit card with my own account, and Steam thought that this was some sort of fraudulent thing? I know I have enough money in my bank and that the card shouldn't have been declined. It isn't anything with my uncle's account either, because my cousin let me try it on his account and the same problem occurs. Yet my debit card works perfectly fine when making purchases on my account.

I've contacted Steam support, but they take forever to reply and the EVE Online sale ends in about an hour. Anyone know why this isn't working?

Edit: I tried removing my debit card as a saved payment method on my account, but it still won't let me make the purchase on my uncle's account.