When you apply a mileage upgrade, the original ticket is reissued, with a new ticket number. It doesn't usually happen right away as it's a manual process. If you did the upgrade close to the 24-hour mark and are trying to check-in before the reissue is complete, then the reservation won't have a valid ticket number associated. If you call reservations they should be able to expedite the ticket re-issue at which point you'd be able to do OLCI. Just one idea as to what may have happened.

But i would certainly try and take care of this over the phone, as ticket reissues at the airport are a crapshoot in terms of how long they take to do.