Quick clarification: katello-agent has nothing to do with errata status. My answer here still holds true. I’m mentioning this to avoid conflating katello-agent and katello-host-tools.
The message you’re seeing: Could not calculate errata status, ensure host is registered and the katello-host-tools package is installed is a little confusing since newer versions of subscription-manager report all of the necessary information. katello-host-tools is not required unless you’re using katello-agent rather than REX to install packages and errata, or you’re on an older platform like EL6 which has older subscription-manager.
That message also shows up if you don’t have any repositories enabled on the host. This is where I think we should show a different message. Even if Katello has the installed package and enabled repo information from the host (via katello-host-tools or subscription-manager) there’s nothing to calculate since there aren’t any enabled repositories.