Clients can't pay

I jus launched a new website, and I have clients getting an error "Can't process payment" when trying to buy a membership. I tried to make them try other cards (From different countries), different browsers, and different devices. Nothing works. Squarespace support escalated my case to a team that is only available via email but I'm not hearing back from them or a I get irrelevant answers. 

Anyone dealt with this before?
Thank you.