Try 8003272177 - I encountered the same issue, no luck on amex site but found this on ticketcrusader site and first code worked on first try,

/how-to-get-american-express-presale-tickets/

When you get to the event page with the American Express Presale, enter the code below in the presale password area and click the lock symbol.
The presale code which unlocks American Express Presale tickets for all events is: 8003272177, 8002973333, and on rare occasion simply “amex.”
Often times you will only need to checkout with your card to get presale tickets. This is called the AMEX FOTL (Front of the Line) Exclusive, but on occasion they won’t be available to select until you enter one of those codes.

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