I want to Add Custom Error Message while User Input Mobile Number or Email id (If its Already Registered Then)

Note: I am Not Using Zoho Creator. i want to do this thing on From Zoho Site form builder.

My Website is Dalal Stock

I read this zoho article ( https://www.zoho.com/creator/help/script/field-actions-on-user-input.html)

and already tried it. but its give error. i am attaching screenshot of it