Ours shifts in/out of all gears just fine. BUT, I'm thinkin one maybe oughta see the R on the gear indicator before getting off the brake and on the throttle. Like almost everything in the computer-controlled vehicle, there is a slight lag whilst the control system "thinks" and then acts on stuff. The mechanical feel and instantaneous gratification is not much of a strong suit with the Smart shifter system. It does function just peachy, but it's " different" .
I can't ever recall getting a "clank" or anything like that.
I'd take a deep breath, check out the shifter operation and note how the control system has a bit of lag... if it indicates yer in reverse and THEN you get some sort of clanking then I'm thinkin you have a slight hangup with one or more of the brakes which makes a noise when you bust it loose. If that's the case I'd talk it over with your friendly, experienced and skillful local dealer type dood.
just a thunk