To define the mirror plane, you must select an existing reference plane or planar face. If there is no suitable plane or face, you can create one with the reference plane commands:
Parallel Plane
Angled Plane
Perpendicular Plane
Create an angled reference plane
Create a reference plane normal to a curve
Create a parallel reference plane
Create a perpendicular reference plane
Create a coincident reference plane by axis (ordered modeling)
