I am looking into AE availability for two seats in November on a YWG-SFO (5 days)-PDX (4 days)-YWG itinerary. AE.com tells me that this is not a permissible itinerary.

The problem isn't obvious to me... I would expect to be able to book on UA or AC for all segments. Could someone please clue me in as to what the problem with this itinerary is? I'm trying to book a west coast vacation with an extra stop and I didn't realize that it wouldn't be allowed.