Custom URL Oops. We are unable to process your request

Custom URL Oops. We are unable to process your request

Hello, i created a subdomain for my mail like mail.simplybest.co.in and pointed to business.zoho.com but it shows  Oops. We are unable to process your request. . The domain simplybest.co.in is working with zoho but i tried to add a subdomain in domains from the control panel but i can't because i am a free user.

Any fixes for this?
just i need to access my mail form
http://mail.simplybest.co.in/