When i type:

Code:
airmon-ng start wlan0 6
i get this error:

Code:
wlan0 iwl3945 - [phy0]/usr/local/sbin/airmon-ng: line 338: /sys/class/ieee80211/phy0/add_iface: Permission denied
mon0: ERROR while getting interface flags: No such device

(Monitor mode enabled on mon0)
Why i get this error?

Thanks