Make sure lsusb can see the wireless adapters (it would show the chipset):
$ lsusb
Bus 001 Device 003: ID 2357:010c TP-Link TL-WN722N v2/v3 [Realtek RTL8188EUS]
Bus 001 Device 010: ID 0bda:8812 Realtek Semiconductor Corp. RTL8812AU 802.11a/b/g/n/ac 2T2R DB WLAN Adapter
Make sure iwconfig can see the wireless adapter:
$ ifconfig
$ iwconfig
$ iw dev
Turn on monitor mode manually:
$ sudo ip link set wlan1 down
$ sudo iwconfig wlan1 mode monitor
$ sudo ip link set wlan1 up
$ iwconfig
Undo:
$ sudo ip link set wlan1 down
$ sudo iwconfig wlan1 mode managed
$ sudo ip link set wlan1 up
$ iwconfig
Or create a separate virtual interface in monitor mode:
$ sudo ip link set wlan1 down
$ sudo iw dev wlan1 interface add wlan1mon type monitor
$ sudo ip link set wlan1 up
$ sudo service NetworkManager restart
$ iwconfig
Undo:
$ sudo ip link set wlan1 down
$ sudo iw dev wlan1mon del
$ sudo ip link set wlan1 up
$ iwconfig
Or do it with airmon-ng:
$ sudo airmon-ng start wlan1
In fact, that does not need to be done as airodump-ng can put the wireless card into monitor mode automatically:
$ sudo airodump wlan1
Make sure, you're not using the default MAC:
$ macchanger -s wlan1
Restart NM when there are troubles with Internet connection: