Zoho Mail Add On is not free?

Zoho Mail Add On is not free?

I am using the Professional version of Zoho CRM for my organisation.  Tried to add Zoho Mail Add On for all of the three users, which is mentioned to be free if one decides to use the Lite version.  However, I am not able to activate it for even one user in the Subscription section.  Does that mean that I will have to pay for Zoho Mail since the Zoho CRM version is a paid one?