Hi I have a A3 2014, This happened to me a few weeks ago the message appeared intermittently but usually when i started the the car they were
Warning brake servo inoperative and
Automatic start/stop: system fault! Function unavailable ... got it booked in at Audi Halesowen who technical staff diagnosed it as the "Brake Servo vacuum sensor which was promptly change and the problem rectifie. Their has to enough of a vaccume in the servo before the Start/stop function works :0
Happy Days