You are trying to access an URL like the following:
and you are taken to a ServiceNow portal login page that you are not familiar with instead of being taken to the record you wish to access.
In fact going via yourinstance.service-now.com/kb gives the same result
Your $sp page is marked as public in the sys_public.list of the instance
Release or Environment
Most probably you are missing a login page from the KB portal.
To overcome this, as the landing page in the login section of the KB portal, as in the below screenshot.