Somehow every theme that is available for free is suddenly displaying this message when I try to check out the cart products (I'm using Venture now). The link points to Checkout settings, but I don't know what to do there.
Since it's happening independently of the selected theme I suppose it's some kind of checkout configuration or universal bug that I couldn't figure out. Don't know exactly how it started, when and why.
Venture's buy it now button still works properly though...