If there are seats available to be selected, then yes, you are able to select seats beforehand.

I see you are an AC elite - was the flight booked through AC (Aeroplan) or UA? I ask because if it was booked through AC, the confirmation number you get will be your AC confirmation number, which UA will not understand.

What you need to do is get your UA confirmation number. You can call AC and ask them what it is - they should be able to provide this to you. You should then be able to login to your reservation on the UA site (using my reservations under reservations & check in) using the UA confirmation number and your last name. You could also call UA at that point, who should be able to assign you a seat as well. Note that if your flight was booked on a codeshare with an AC flight number you will have to call as the seat map won't show up online. But IME, reward tickets usually book as the operating flight, not as a codeshare.