As a new user I am working my way through the various settings.
In "Knowledge Base Settings and Options" I tried to tick 'Require client login' and get the error message: The knowledge base cannot be restricted unless client registration is enabled.
The client login setting is set to 'Private' which means that registration is by agent setup but a login IS required.
If I change the setting to Public then there is no error.
A work-round is to set it to Public, change the option and then put it back to Private.