Whmcs Integration not working with modern checkout

When I select modern as the default order form template in whmcs everything works fine other than the checkout button. You can check it out here http://stage.discountwebdesigner.com/dwd-wp/dashboard.htm?whmcsportal%5Bpage%5D=whmcss%3A%2F%2Fwww.discountwebdesigner.com%2Fclients%2Fcart.php

Once checkout is selected then the browser gets redirected to http://stage.discountwebdesigner.com/dwd-wp/dashboard?whmcsportal%5Bpage%5D=whmcss%3A%2F%2Fwww.discountwebdesigner.com%2Fclients%2Fcart.php%3Fa%3Dcheckout which is a 404 page.

Any idea as to why this is occuring? When I use other default order form templates things seem to work fine.