I am not able to add my domain name for Zoho Business email

I am not able to add my domain name for Zoho Business email

Hi
I hope you are doing fine.
When i m trying to add my domain name for the Zoho Mail, it is getting me the error message "This domain is not allowed to add in Zoho. Please contact support-as@zohocorp.com for further details".
I am not using a free domain name. Please assist me.
Thanks
Foni