[Survivor]

I know this is an old post but if anyone happens to view it, this is how codeshare agreements work. The carrier that issued your ticket is responsible for all aspects of the service. UA contracts with Air Canada to sell tickets on our flights, seat selection may or may not be something they choose to do. UA has the ability to do seating where they provide it. It's much easier to tell you to go to the operating carrier for seats because it makes sense - but we have set aside seats for them to use and we don't have access to them. Within 24 hrs ALL seats are available to everyone when you use Web Checkin for the carrier operating the flight. Get a record locator for the operating carrier and use their Web Checkin (in this case AC) on their website.