One problem related to transmission not engage has been reported for the 2016 Audi S6. The most recently reported issues are listed below.
There is something wrong with the way the S6 downshifts which makes driving unsafe. A downshift is followed by about 1 second of the transmission not being engaged (so no power to the wheels) and then suddenly the transmission engaged and the car violently thrusts forward. The issue is more profound when downshifting 3rd to 2nd at about 20-30 mph and can result in rear-ending the car in front. Apparently many S6 owners face the same issue. Audi has fixed it in 2013 models with software update but the issue happens again in the latest models.