American Express Reason Code P01

What is

The card number charged isn't a valid, assigned account number.

A data-entry error or card-testing fraud attempt used a card number that doesn't correspond to a real account.

Often a sign of card-testing fraud activity on your checkout, worth investigating beyond this one dispute.

Your checkout's card-validation log at time of purchase.

Submit your validation log; this is rarely fightable if the number truly wasn't valid.

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Not running basic card validation (Luhn/format check) before attempting authorization.

Add card-testing detection (failed-auth velocity limits) to catch this pattern before it recurs.