So I am about to transfer my Chase UR from my chase sapphire preferred to British Airways Avios, since I just got my bonus.

I need to book a flight from BOS-ORD for Aug 11-14, 2016. I checked American Airlines's website and there are flights for this trip under the MilesAAver awards.
I then head over to British Airway's website and plugged that that trip under Book with Avios and it say there are no flights available. So I am a bit puzzled by this.

Are the MilesAAver flight on AA's website not bookable with Avios?