I believe that the message in your screenshot is expected when two operators (or the same operator in two different requestors) attempt to trace a service rule. Ending the other session should allow you to trace.
I have frequently seen this on our in-house systems. For some reason, a previous Tracer process never died even though the UI stopped showing it; and now you have two. I've had to wait a few hours until the process was automatically cleaned up. Frustrating to no end. I've never tried to find the Requestor running the Tracer and kill it -- not sure how easy that might be.