Hi in HFSS, once you define a port, you go to "Analysis" found in "Project manager" located at the left. Right click and "add solution setup", provide your solution frequency. Then a "setup1" will be instantly created. Then right click on "setup1" and "Add frequency sweep". Define your start and end frequency. Then run simulation (provided you completed other steps). After simulation, right click on "Results>create modal solution data report>rectangular plot> s-parameter" . There you have it , Select s11 or s21 and select dB under "function". Then, press "new report". Then the plot will be generated