I checked your site and it’s clear that the Registration Checkout page is not correctly set up. Here’s how you can fix the Registration Checkout page:
1) You go to Event Espresso > General Settings > Critical Pages
2) Check the page set for “Registration Checkout”. If either Page Status or Shortcode isn’t OK, then you can either add a new “Registration Checkout” page on your site, or update an existing page on your site. The page must have this shortcode on it:
After the shortcode is on a published page, you can change the setting to point to the correct page.