There are two different types of mirror operations in Studio.
One is the Exact Surfaces>Template>Mirror Template.
The Mirror Template can only be used when a NURBS surface is not present. This is because the template is used to generate a new NURBS and it would need to be removed if you wanted to create new NURBS surface. A way around this would be to duplicate the object in Tools>Create>New Object>Duplicate and then create a new surface using the mirrored template.
The other type of mirror is in the Tools tab called Mirror Model.
This creates a symmetry by duplicating the structure of an object on the other side of a mirroring plane.