Hi Jack, sorry you're having trouble. You're the first to report that issue. I'm not sure what the cause would be. We've had one other customer have trouble placing an order but he was having issues with adding to the shopping cart.
The first thing I would recommend trying is a different web browser and or clearing your browser's internet cache. If you continue to have trouble please pm me with details about the web browser you are using, type of computer (mac, pc, phone etc), and the time you most recently got the error message.