It means HFSS is not able to mesh the object it happens mostly when due to some Boolean operations model has infinity thin edges which makes meshing impossible in that region. The best way to check this is try to generate the mesh only instead of running the simulation and see if you get error? If mesh is generated just try to see if it is proper when I say proper it means if it is uniform across the object. It is always better to check the mesh before going for simulation.