Amex plat credit not working

Hi, I have a friend with Amex Plat that isn’t an Equinox member. He wanted to put his card on my account to pay for my membership and then get the $300 credit, but the Equinox Amex portal rejected the card saying that the card has already used up the benefit. This happened with another friend of mine’s card as well. Any tips on why this might’ve occurred? One of them is a supplemental card but we are sure that nobody else used the credit.