If your trying to capture a handshake you need a client connected to your target AP and then you need to run in a terminal shell
aireplay-ng --deauth 0 -a <mac address of target AP>
once the client has been disconnected stop the deauth and wait for the client to reconnect to the AP and hopefully you will capture the handshake then run aircrack-ng to decrypt the handshake
Hope this helps and i also hope that you are testing this in your own lab environment as cracking other peoples WiFi is illeagle
--Chard



