I'm having trouble with the GOG checkout pages. This is happening on desktop browser as well as my iPhone browser (Safari).

When I get to the Payments page, I select a payment method but it doesn't allow me to continue. The "Pay for this order now" button is grayed out and unclickable. I cannot figure out how to proceed.

I've tried selecting Apply Pay as well as Credit Card but the button is always disabled.

There is a message saying "please review order to proceed" but I have no idea what that means.

See attached screenshot.

Aye, that screenshot doesn't tell us anything.

Doh! Nevermind. Turns out I had an item in the cart that was already in my library. So, it was preventing me from purchasing it again. Which is nice. But, they could have given a better message.

Are you trying to buy a DLC without owning the base game?

Shoot a ticket to support if you think that's a problem you'll face in the future.