Merchant lacked a valid authorization.
This is an authorization-category code: the transaction was processed without a valid, unexpired authorization. The dispute is decided on your authorization records, not on fulfilment.
Produce the authorization log: approval code, timestamp, and amount, plus proof that capture happened inside the authorization window. If the issuer approved the transaction, say so in the first sentence of the rebuttal and attach the auth record.
You typically have 20 days from the dispute date to submit your evidence package through your payment processor. Missing the window forfeits the dispute regardless of merit, so assemble the checklist below as soon as the dispute arrives.
This code maps to Amex MOG A02. Confirm the current wording, timing, and document requirements with your processor before submitting - networks revise their guides and your processor may impose shorter internal deadlines.