There are two ways to enable monitor mode for wlan

USING IFCONFIG AND IWCONFIG
iwconfig to check mode of wlan0
ifconfig wlan0 down to disable wireless interface
airmon-ng check kill to kill processes which can interfere with it
iwconfig wlan0 mode monitor to change to monitor mode
ifconfig wlan0 up to enable wireless interface again

USING AIRMON-NG
ifconfig wlan0 down
airmon-ng check kill
airmon-ng start wlan0 
ifconfig wlan0 up