Looks like you're trying to access the configuration page directly. This page can only be accessed by clicking on the Build your own button from the product or the shop page.

— You can purchase this kit in multiple quantities on the cart page AFTER adding it to your cart. The add on’s you select will only be added to the first configuration; which means we will need to add them to your cart after you check out. This is a short term workaround for a current bug in the software. You may disregard this notice if you are only purchasing one of these items. —
— Please be patient when adding kits to the cart. It may take a few seconds. Thank you! —