How to configure WiFi adaptor to monitor mode?

It may work if you first bring the interface down:

sudo ifconfig wlan1 down
sudo iwconfig wlan1 mode monitor

Check:

iwconfig

Not every device and driver combination are capable of monitor mode.

The best way to put your Wifi adapter to monitor mode is through airmon-ng.

You can install it by typing into a terminal sudo apt-get install aircrack-ng

Also, if you are using Wireshark, you have to run it as superuser to be able to put your adapter to monitor mode... :)