I would search the internet for this error message. Thanks to
https://askubuntu.com/questions/8413...of-wifi-driver, I know now that the command
iw list can be used to confirm whether an interface is able to monitor.
But first of all, your setup is not quite clear to me. eth0 is certainly a strange name for a wireless interface, and you also refer to "Ethernet"; are you sure it's really wireless? Also, while I am not sure if it's relevant, I don't understand your remark about the router.