You can't make seat selection on ac's website . However, you could request aisle seat on cheapoair's website once ticketed . After requesting , seat assignments will be automatically generated and shown on your itinerary on aircanada.com -> manage booking .
Does AC permit you to do seat selection now on this fare? Both "L" and "S" are good for EQM/EQS Credit on UA according to the UA FAQ. The only hitch with me is if I have to sit somewhere other than an aisle.
How did you (or did you?) make your seat selection?
Thanks,
SST