Occasionally the car won't shift into any gear but neutral and Park. It's not very common but I use it for work so I need to figure out what the problem is. I just spent a whole bunch of money to get a whole new shifter and it's still doing this.
Make sure that your car has the two software updates that primarily addressed the issue of not shifting into reverse. The clutch actuator should be adjusted using a Star machine. You may need to visit a Mercedes-Benz Dealer that services smart cars or find an independent German import car repair shop that has a Star Machine.