First get Target AP's client connections using the commands in this reference:
Open a second terminal and disconnect a client from the AP by executing the following command. This will allow one to capture the handshake between the AP and client. Handshake will be in the generated pcap file.
aireplay-ng -0 3 -a <Target AP's bssid> -c <Target AP's Client's essid> <wlan name>
Note: the -0 3 is telling the command to execute three times, you can increase the three to a larger value to keep client from reconnecting.