Active Card & Monitor Mode

Set your WiFi card into monitor mode with airmon-ng

You might want to stop the Network Manager

systemctl stop NetworkManager

or disable the NetworkManager from autostart after reboot

systemctl disable NetworkManager

There is a possibility to limit Network manager devices but for Wifi hacking you kill all processes which interefers with Wifi.

Network Manager

Card Detected

After reboot, your card is detected but not enabled at the moment.

ifconfig does not list the device wlan0

ifconfig

until you use (ifconfig -a) which lists also inactive cards.

ifconfig -a

iwconfig lists your wifi card all the time, regardless of it's active or inactive.

iwconfig

wlan0   IEEE 802.11 ESSID:off/any
        Mode:Managed Access Point: Not-Associated Tx-Power=20 dBm
        Retry short limit:7 RTS thr:off Fragment thr:off 
        Encryption key:off 
        Power Management:off

We need to activate (enable) card for use.

Enable Wifi Card

ifconfig wlan0 up

It might happen that your card is soft blocked by the kernel.

SIOCSIFFLAGS: Operation not possible due to RF-kill

Perform the following command and enable the network interface (wlan0) one more time.

rfkill unblock wifi

# try one more time
fconfig wlan0 up  

airmon-ng - List available interfaces

airmon-ng            

PHY     Interface       Driver          Chipset
phy0    wlan0           88XXau          Realtek Semiconductor Corp. RTL8814AU 802.11a/b/g/n/ac

Enable monitor mode

airmon-ng start wlan0 

Found 1 processes that could cause trouble.
Kill them using 'airmon-ng check kill' before putting
the card in monitor mode, they will interfere by changing channels
and sometimes putting the interface back in managed mode

    PID Name
    440 NetworkManager

PHY     Interface       Driver          Chipset

phy0    wlan0           88XXau          Realtek Semiconductor Corp. RTL8814AU 802.11a/b/g/n/ac
                (monitor mode enabled)

You can see in my case the NetworkManager with PID 440 was detected as interference, kill or disable the process.

when you are in a monitor mode, the iwconfig command shows Mode:monitor

iwconfig
lo        no wireless extensions.

eth0      no wireless extensions.

wlan0     IEEE 802.11  Mode:Monitor  Tx-Power=20 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:off