Display an error when user is not logged into the community and Is Visible In Pkb == false in Knowledge Article Version in Salesforce

[Jogender]

If the PKB checkbox is checked on the Knowledge Article, it means the Knowledge Article will be visible to unauthenticated users. So if in your case IsVisibleInPkb == false then the Knowledge Article should not be visible to the user if he or she is not logged into the community.

So there should not be any need to display the error if the Knowledge Article is not at all visible to the user. Hope this makes sense.