Why does your Audi A4 3.0 v6 not start when you've have your battery and alternator checked and both are fine?

So many things could be wrong and cause it to not start, but from what you have provided the next step would be to trace the cables to the starter and see if they are corroded or broken, you didn't mention if the car was turning over or not, so I assume it isnt, so if the battery has power, and the alternator isn't the cause then you need to follow the power, it goes from the battery to the ignition, then to the starter, since we know the battery is good, check to see if power is getting to the starter, if not trace the problem in between those two points.