If you want to do a 2D structural analysis, for example plane stress, plane strain or axisymmetric, you must construct a surface in the XY plane in SpaceClaim.  For Axisymmetric, the surface must be on the +X side of the Y axis, which is the axis of rotation.

In Workbench, you must change the Analysis Type to 2D in the Properties window under the Advanced Geometry Options BEFORE you double click on Model and import the geometry into Mechanical.

If you forget to do that and build the model in Mechanical as 3D, do not go back to Workbench and try to change the Ansysis Type to 2D.  Some objects will be 3D and will not work properly in a 2D analysis.  It is best to just start over.