I usually see that on network-manager when a device is hard/soft blocked by a switch or rfkill, or when the device is listed in /etc/network/interfaces, or when the device is set to unmanaged by .conf file somewhere in /etc/NetworkManager
I guess there could be other reasons that it is unmanaged, the best way to find out is probably to read some documentation.
If you are not using networkmanager, then read the documentation for whatever gui tool you installed, or try some websearches.
https://wiki.debian.org/WiFi/HowToUse