PayPal code + Squarespace

Site URL: https://www.fnt-design.com/payments

Hello– I'm trying to set up the following checkout page to work with Paypal: https://www.fnt-design.com/payments

I've connected my Paypal business account with Squarespace successfully. Then I'm using this: https://www.paypal.com/buttons/smart (Set Up Your Paypal Checkout Experience) to generate some code. Finally, I am pasting the code into my Squarespace site using a code block.

As you can see from the link, the checkout appears correct, but when I try to test it with a credit card payment, it's rejecting every card I try ('We're sorry, but we weren't able to add this card. Try again later or add a different card.') 

Did I miss a step?

Thanks!