i have aircrack installed and i did what u said
spaik@ubuntu:~$ sudo airmon-ng start wlan0 1
Found 5 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!
PID Name
1139 avahi-daemon
1140 avahi-daemon
1219 NetworkManager
1301 wpa_supplicant
4046 dhclient
Process with PID 4046 (dhclient) is running on interface wlan0
Process with PID 8175 (dumpcap) is running on interface wlan0
Interface Chipset Driver
wlan0 Atheros ath5k - [phy0]
(monitor mode enabled on mon0)
so what does that mean..
and when i try to test the injection
spaik@ubuntu:~$ sudo aireplay-ng -9
No replay interface specified.
"aireplay-ng --help" for help.
i know i am still learning but i want to learn about this issues and maybe work in the security field so thanks for ur time :)