I had this issue all day yesterday as well. I tried again today and still get the same error. I have tried with Chrome, IE, and Firefox and on 3 separate Windows computers.
The flight I'm trying to book is a roundtrip ticket from DC to Ottawa operated by American Airlines. I called Iberia yesterday 4 different times and spoke to 3 different reps, all said that they do not see the reservation on their end because it is operated by American. Which makes no sense since I am using Avios. I explained to them that this is an award ticket. They insisted that award tickets operated by American cannot be booked from their side of the system... They suggested I call American. Which I did, and their reps were just as confused as I am as to why Iberia thinks I can book Iberia reward tickets by calling American. So this has been fun.