Error on Order Received – Undefined Variable

When users complete a checkout, the bottom of the order received screen shows: “There has been a critical error with your website”.

Checking logs, it looks like it could be due to the undefined variable “WCZELLE_PLUGIN_VERSION” in thankyou.php.

Please feel free to let me know if you need any more info. Everything still works fine so I think the error is more of a warning, but some customers are understandably alarmed after completing an order.