Quote Originally Posted by lat9 View Post
You might have some other checkout-related observers installed that are "not helping". Please list the files you've got in your store's /includes/classes/observers folder.

Once you've uploaded the files to your store's file-system and click that "Admin Home" button, you will see the Admin->Configuration->One-Page Checkout Settings menu item which, when clicked, shows you the plugin's current settings. No message is issued.
One-Page Checkout Setting

Version/Release Date 1.0.1 (2016-08-06)
Enable One-Page Checkout? True
Enable One-Page Checkout debug? False
Debug: Customer List Nothing showing