The same methodology for PCB symbols is used with Schematic symbols. Before you can use your new PCB symbol in a design you have to produce a component even though this component may not have a Schematic symbol associated with it. A PCB only symbol could be added to the design as a drawing blank or a test-pad probe point for example.
Once a PCB only or Schematic only component has been created, the missing opposite symbol type can be added to make it available to both design editors.
How can I use the Library Manager?
How can I manage embedded libraries?