I can see by your photo that your aircraft is already on the runway. Since you don't need taxiing instructions, tuning to 121.70 does nothing. Try starting at the gate and the whole process should work.
Alternatively, you can also use the Nearby Air Traffic Controllers window. You can toggle it on and off from the Special menu. It will indicate which controller frequency is recommended for where you are in the process and you can auto tune your radio by clicking the line.