I booked a flight on US using Avios. I then went to the Finnair site to enter my AA number so that I could get a preferred seat assigned. Fast forward a couple months. I now need to cancel the flight. When I input the BA PNR, I'm told that BA doesn't recognize the PNR. I went back to Finnair and input my BA number. I returned to the BA site. Still no joy. "Sorry, we are unable to add bookings that do not contain any British Airways flights to your account."

Given that the last time I called BA I was on hold for 90 minutes before giving up, what should I do? Is there a way to do this online?

If there is already a thread covering this, please give me the link to it.