The "Knowledge One" permission set needs to be assigned. In order for users to be able to switch from the Articles tab to the Knowledge tab, the Knowledge One permission should be added to the permission sets. To do this, "Permission Sets" should be entered into the "Quick Find" box.
From there, you can select the permission set that you want to add. You can search "Knowledge One" and edit it. You need to check the "Enabled" check box for Knowledge One in "Knowledge Management." Once you click save, your users will be able to view Knowledge.