1*801*658*0201 How does American Airline Seat Selection work?

Taking a US domestic American Airline flight with general seating, Everything I've read said we can select our seat at booking but this was not an option. Calling AA, they said All the standard seats were blocked and the seats would be selected by the airline unless we wanted to pay for an upgraded seat. But he also said that we could select seats and didn't know why they would all be blocked.
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