2D sheets can't be assigned as materials for anything but finite conductivity boundaries. If you are selecting the faces of the pyramid as a port that is what is causing the port assignment error. like the error says only faces in the same plane can be assigned as a single port. I think the outer faces should be PEC boundaries. Port should be at connector on pointy tip of the cone. you can use a non model plane or object internal to plot fields at the DUT as an option.