A ::CLOUDFLARE_ERROR_500S_BOX:: token was not detected on the error page

What is the name of the domain?

ordertracinventory.com

What is the error message?

A ::CLOUDFLARE_ERROR_500S_BOX:: token was not detected on the error page, and must be added before this page can be integrated into Cloudflare.

What is the issue you’re encountering

Error creating a custom error page

What steps have you taken to resolve the issue?

I have created a custom error page using the exact template in your documentation. Here is the error page:
http://ordertracinventory.com/appmaintenancemode.html

I am using the example found here:

Yet when I try to save the error page, it gives me this error:
A ::CLOUDFLARE_ERROR_500S_BOX:: token was not detected on the error page, and must be added before this page can be integrated into Cloudflare.

What are the steps to reproduce the issue?

Go to Custom Pages, click Custom Pages next to 500 Class Errors. Paste in my custom error page url found above. CLick Publish… I get the ::CLOUDFLARE_ERROR_500S:BOX:: token not found error even though that text does exist in the error page.

Screenshot of the error