Originally Posted by ckpeter

Because the OP has UA miles, which can only be used to book the "saverpass" level of CO award seats, not the standard "easypass" award level.

The best way to check CO award seats is to go to CO's site and check their award availability. Any awards available on the saverpass level should also be available to book by calling UA too. UA's site is supposed to show CO flights, but if it doesn't show, it may be because there are no awards available on the CO flights.