To create internal symmetry, the piece must be symmetrical to start with.
To create internal symmetry:
On the Main toolbar, click Select.
On the Horizontal contextual menu, ensure Pieces is selected.
In the 2D window, click the piece on which you want to add symmetry.
Go to the Context view, in the Actions section, at Inner Symmetry.
Click Create. A line showing the symmetry is displayed in the piece.