This error is generally encountered on all A- series platforms (except A304/A604) and suggests that the 3420 downlink signal is not reaching the Fader's Channel input N-port (Channel 1 in).
1. Verify that your RF cables are connected properly according to the Rack Setup Guide. Access the rack setup guide through the Test Manager title menu Help-> Platforms-> Axxx -> Rack Setup Guide
2. Verify that your RF cables are adjusted with the torque wrench
3. Exchange RF cables if necessary if all the above has been verified