Standard award travel on United is available based on the full fare booking class for that cabin, ie F C or Y.
Upgrade seats are based on availability in NF and NC and these are given less availability than the standard award.
Saver awards, depending on the cabin, are in general given less availability than the upgrade seats.
So it goes Standard award availability, then upgrade availabilty then Saver award availability. So it is always possible to have award availability but no upgrade availabilty.
Hope this helps