This is a known problem flying Iberia when it isn't booked directly thought IB. I has this problem a few weeks ago where I booked ORD to MAD on an AA codeshare. It gave me the same message. I'm EXP. I just couldn't select a seat. AA twitter couldn't help me. Luckily my connecting flight was delayed into ORD and got rerouted JFK to MAD. But IB has an awful system. Even the Finnair/RJ trick doesn't work sometimes.