Since it is hard to troubleshoot the problem without knowing more details (flight number and date i.e.), I would call UA and just try to book it over the phone. If they can't book it, they can probably tell you more.

It may also be that you are trying to book a flight to far in advance. While schedules are already available it may be for several reasons that UA does not want to accept bookings if it wants to work out further details (revenue sharing and what not) with the airline they are codesharing.