Originally Posted by
californiadreamin'
You may choose to blame AC, but the UA agent has to know how to do this and not refer you to AC. This has been discussed over the years ad-nauseum: No matter how much goodwill, effort and seeming brilliance or kindnesses by AC agents, you will not be able to confirm a seat on AC metal when booked as a UA flight number UNLESS you find a UA agent who knows how to generically request (aisle or window, for example) a seat for "their" flight via "their" PNR; and even then the response is not instantaneous. If you are told otherwise, you may feel placated and happy, but be prepared for possible disappointment when you checkin (if it truly matters to you for such a short flight).
My interpretation is the OP booked through AC, on Aircanada.com and has seats on the AC-operated, AC-coded YYZ-ORD leg, and wants seats on the UA ORD-SFO leg (AC flight number, I assume), but can't do that online. However, this can definitely be done by UA (not sure if AC can do this).
I know that I was recently able to do this, booked on AC flight number but UA equipment, called UA to get E+ assignments, as AC could not provide this. Seats were assigned instantly, as I checked my itinerary on UA.com right after the call, and my E+ seats, as described on the phone, were selected. So this can definitely be done.