Try this:
BT ~#airmon-ng start ath0
BT ~#airmon-ng start wifi0
Then run iwconfig and check if ath0 is in Monitor mode. If it still isn't, try the following:
BT ~#ifconfig ath0 down
BT ~#airmon-ng start ath1
wifi0 should now parent ath1, and ath1 should be in Monitor mode. If it isn't, try:
BT ~#airmon-ng start wifi0




