The Cause
(1) The HDMI-HD/AV transmission module is disconnected from the camera.
(2) The camera is powered off.
(3) In HD mode, the DJI Lightbridge App video source has been set to “HDMI/AV”.
What to Do
(1) Check HDMI-HD/AV transmission module connection.
(2) Power on the camera.
(3) Change the DJI Lightbridge App video source to "HD Gimbal".