Agree with GB.

Not sure how this would work as an AA-booked (economy) award flight; however, normally if booked on a partner's (paid) ticket stock, you are SOL for seat selection.

Regardless, you will need to pay for any seat selected unless the passenger has Enrich Platinum / OWE equivalent status - though, this free selection also would not apply to premium seats (i.e. exit row / front bulkhead seating).