Glad you were successful Susan! Anyone else reading this, once your tickets are in your basket, make sure to click on the icon next to your ticket that says "Warning: missing some fields" and enter your first name and last name there. Then scroll down to the bottom and put a checkmark in all three boxes (General Terms, Regulations, Nonrefundable) before you hit confirm, otherwise you'll get another error. Once you've clicked confirm and advanced to the payment screen, click on GESTPAY and then enter your credit card details.
[edited to add: if there are multiple people in your party, I think you have to click on the icon for EACH ticket in your cart and enter that visitor’s first and last name. There’s a link on the website to notify them (required) of any name changes to who is visiting.]