Please try to re-enable monitor mode again using this:

ifconfig wlan0 down
airmon-ng check kill && rfkill unblock all
iwconfig wlan0 mode monitor
ifconfig wlan0 up

– Make sure you do NOT connect the adapter until Kali fully loads, and after you log in as root,
– Connect it directly to the computer, don’t use a hub or an extension, and
– Try different USB ports in your PC with different USB settings (2.0 & 3.0) in Kali USB settings in Vbox.

If the above didn’t help, then try to enable monitor using another method:

If all of the above didn’t help then try updating Kali using the following commands
apt-get update

Hope this helps.