Currently, the transient solver doesn’t support non-TEM mode ports. So TE & TM port modes for waveguides for example won’t work, and neither will quasi-TEM port modes for structures like microstrip lines and coplanar waveguides. Only TEM are supported. A workaround for microstrip and coplanar waveguide structures would be to use a lumped port between the signal conductor and ground. Do this, instead of using a waveport, whose geometry would extend to the air above the microstrip line (and touching multiple conductors), thus supporting quasi-TEM port modes. The lumped port will support purely TEM modes, thus getting rid of this error.