I had the same experience on my recent American Airlines flight as a 75K. I entered my Mileage Plan number when I made the reservation and was able to select a Main Cabin Extra seat. Checking in on the app showed a charge for adding a bag so I called AA customer service and they told me that I had to check my bag at the airport to get my free checked bag benefit. I had no issues checking my bag for free at the airport and I even got upgraded to First Class on my way to the gate.