Why the mirror modifier is not applied on my half-cube? I've applied a mirror modifier on my half-cube but even though the pivot point is centered correctly (in the center of the half cube) in both object mode and edit mode. It is still not mirroring my half cube. Why is it not working?

The pivot point in object mode:


The pivot point in edit mode:
You are mirroring in the X direction, while your half cube is oriented along the Y direction. So the mirror works, but the generated vertices coincide with existing vertices.