Recently, the cPanel interface of existing Bluehost's Shared Hosting has been changed the new interface. Now, it's easier to manage WordPress installations.
In the new cPanel interface, no SSL warning message appears for each domain under Domains > My Domains:
However, when you go to Advanced > Security > SSL/TLS Status, you will find "AutoSSL Domain Validated" for each domain.
For this issue, Bluehost said:
Since the SSL is installed for all the domains on the account you can ignore the message in the Domains tabSource: Bluehost Support Team
So, you can just ignore the message under the Domains tab.
If you add a new domain as an addon domain, Bluehost can install SSL for you.
If the domain is pointed to us we can install it for you on the domainSource: Bluehost Support Team
When you add a new addon domain and update the name servers of the domain to Bluehost, you can contact Bluehost for installing SSL for the new domain.