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I am getting a For frame [/cam_frame]: Frame [/cam_frame] does not exist.

this implies the sensor_msgs/Image messages have their header.frame_id set to cam_frame, but that TF frame is actually not broadcast by anything.

The Camera display shows "the world" of RViz rendered from a specific viewpoint. To be able to do that, it must "know" where objects and the camera frame are. For this it uses TF. If you specify cam_frame as the frame_id (== TF frame), but that frame is not broadcast by anyone, RViz cannot resolve the location of the viewpoint from which to render objects.