I booked an AAdvantage award ticket (I) and had no problem booking a chauffeur today using their web form. They have confirmed it by e-mail. You do have to wait until after the award is ticketed by AA; neither seat assignments nor chauffeur bookings can be made before that. I don't know if that is why you were 'denied'.

The seat assignment thing is a pain because the AA agent cannot do it when making the reservation; and it cannot be done on EY's web page even after ticketing. I had to call EY.