Demiplane not letting me add my card or change my billing address

I’m trying to buy the VTM player’s guide on here but the website won’t accept my card for some reason. Despite the details I’m putting in exactly matching what’s on my card, I keep getting errors and it says my zip code is wrong. When I go into my billing information and try to change my zip code, it immediately reverts back after changing it. Why is it not letting me change my zip code? Why am I experiencing errors?