The APU is able to deliver +/- 1.2kg/s bleed flow.
The 2 air conditioning packs need +/- 1 kg/s bleed flow.
The 2 WAI valves need +/- 0.7 kg/s bleed flow.
Therefore, as the APU and the engine cannot deliver together bleed to the aircraft, if the APU is selected for the bleed, the APU is not able to deliver enough bleed flow for the Packs and the Wing Anti-Ice.