Error Message: "Not found on generator. request on host not found."

From the membership levels page (http://keithalpichi.com/membership-account/membership-levels/) I’ve tried testing out one of the three checkout buttons and am redirected to the checkout page (https://keithalpichi.com/membership-account/membership-checkout/?level=1). However, it’s blank and I’m given the message:

“Not Found on Accelerator
Description: Your request on the specified host was not found. Check the location and try again.”

What is the issue?

https://wordpress.org/plugins/paid-memberships-pro/