We are aware of this issue and we apologize to all our customers who've experienced it.
To avoid this, please use only Google Chrome browser (avoid Internet Explorer or Edge), and this should resolve most of the issues when you are unable to submit an order.
Also, when checking out with your credit card, make sure to use the credit card details exactly how your card has them, same name, same address etc.
For the time being, please avoid using American Express. We are looking to solve payment errors with Amex as soon as possible.