I just booked an avianca flight with Aeroplan points and I seem to be running into the same problem. AirCanada website says that assigning seats is on Avianca while Avianca says its on Aeroplan. So I will try the chat function with Avianca too, but not optimistic. May just cancel the booking if I can't get it resolved as I'm flying with wife and infant son, so don't want to take chances of not having the seats together.