I thought there was some way to use AA miles to upgrade BA operated/marketed flights, but don't recall the details.
The cross-over here on the operated/marketing may be the problem, in the end.
Cheers.
Yup:
https://www.aa.com/i18n/utility/jba-...servations.jsp
"In order to upgrade on a flight operated by British Airways or Iberia,
your original ticket must be booked directly with American Airlines (via Reservations or AA.com)."
I read the OP as having been booked outside of AA.
OP: How was the ticket booked? Via AA (AA.com, AA reservations), or in another manner?
EDIT:
sukn beat me to it.