This is happening because kojid is authenticating as a user that is not associated with a build host (or perhaps not authenticating at all). You can't run kojid as a regular user (or anonymously). The hub is trying to determine which host is calling the function and is not finding one. Certain calls can only be executed by build hosts.

When you created the host entry (koji add-host ...), an associated user was created with the same name. This is the user that kojid should authenticate as.

And just to clarify, the users I'm talking about are the ones internal to koji, not the system users.

Thanks. However, how can I associate the kojid user with a build host? Does it mean that I have to configure a DNS to make all this names associate together? Or is there something that I need to configure which I may not have done?

At the beginning of wiki "https://fedoraproject.org/wiki/Koji/ServerHowToProposed", it is mentioned that - "
Builders
     By default these will be referred to as kojibuilderX, but can also be the hostname(s) of the boxes that will be setup as builders. TODO: can also or should ?
"
I think this may be why I can't make it correctly. What I'm confused now is what the mentioned "hostname(s)".