It means there are no exit nodes in Greece right now or you need to make sure you have StrictNodes 1
turned on in conjunction with the country setting or there are ExitNodes in Greece but they are not reporting as such (ExitNodes {??}).
So make sure your torrc has StrictNodes 1
and ExitNodes {gr}
and report back if that works. Case might also matter. Haven't checked if it does or not, so GR vs gr.