You were trying to book by using the online tool, yes? It's completely unreliable. Apart from errors which shouldn't occur but which prevent you from making the booking, it also doesn't have the rules programmed in correctly; so itineraries which are valid by the rules aren't permitted in the online tool. You'll be much better off making your booking by phone. I usually use the AA RTW Desk in the U.S. +1-800-247-3247. They know their stuff and will try to help you, too.