I am new to ubuntu os.I have installed aircrack successfully and trying to execute the  command  sudo airodump-ng mon0 but its not working.How to fix this issue ?
Error:
ubuntu@ubuntu:~$ sudo airmon-ng start waln0
Found 4 processes that could cause trouble.
If airodump-ng, aireplay-ng or airtun-ng stops working after
a short period of time, you may want to kill (some of) them!
-e 
PID Name
1382    avahi-daemon
1383    avahi-daemon
1433    NetworkManager
14727   wpa_supplicant
Interface   Chipset     Driver
eth2        Unknown         wl
ubuntu@ubuntu:~$ sudo airodump-ng mon0
Interface mon0: 
ioctl(SIOCGIFINDEX) failed: No such device
also tried with eth2 interface but no luck!
ubuntu@ubuntu:~$ sudo airmon-ng start eth2
iwconfig
ubuntu@ubuntu:~$ iwconfig
lo        no wireless extensions.
eth0      no wireless extensions.
eth2      IEEE 802.11  Access Point: Not-Associated   
          Link Quality:5  Signal level:0  Noise level:0
          Rx invalid nwid:0  invalid crypt:0  invalid misc:0
ppp0      no wireless extensions.