For those familiar with Membership Rewards, it's the only thing OP could be trying to do. AAdvantage is not a transfer partner with Amex Membership Rewards.

To OP -- it's possible Amex/Orbitz is selling a Basic Economy ticket in which case you would have no seat assignment. If it's a normal Main Cabin fare, once you have been ticketed you can log into AA.com with your record locator and choose a seat. Note that if Amex isn't offering one, it's possible that all remaining seats are "preferred" and thus cost money if you have no elite status. If that is the case you can keep the ticket with no seat assignment and all the 'preferred' seats become available at OLCI (just be precise at the T-24 hour mark).