Select the plane that will support your sketch :

In PartDesign WB, you will start your construction from a plan drawing (sketch) and then add volume to your construction.

To select a construction plane, simply click with the mouse, either on the graphical representation of the plane, or on its name in the construction tree :

          ou           

If your FreeCAD experience allows you, if necessary you can also create a specific reference plane.

To select a construction face (when the topological naming problem has been resolved, and obviously if material already exists!), All you have to do is click with the mouse on this face, on the 3D view of the object :