Once Mechanical has attempted an Attach to 3D geometry, you cannot later change the Workbench Geometry Properties from 3D to 2D. If you try, you will get an error message.

Start a new Static Structural analysis in Workbench, click on the Geometry cell and in the Properties window, set the Analysis Type to 2D.  Then import the file you have created already with a surface in the XY plane at Z=0.