Hi ..... adding material properties after your mesh was created does not help. You need to associate the material properties with all elements. In that case you have two options - either you clear your mesh, select material you defined, suitable element, and remesh your model  or you need to modify the attributes (material) associated with your existing elements (if this option is available in Workbench).    

Thank you all for the help!  The error indeed was due to meshing prior to material definition.