I am trying to reserve seats for JAL flight booked as BA flight on BA.com.

I used to reserve seats without issue for flights booked via BA by using JAL's portals Reserve Seats page. But it does not find my reservation for this booking.

Is it because the flight is booked as BA flight (showing BA flight number on the booking for JAL metal) ? I enter the JAL flight number equivalent JAL48 with the BA booking reference but it throws an error.