However, Amex accepts online chargebacks even though it is not required to do so. OP's problem is that the law only requires 60 days. I would call Amex and try this by phone, noting that it is for an air ticket where the flight has been cancelled. If Amex still will not accept it, but all means send the chargeback request in by mail (details on your statement), but don't expect much. You are then relegated to small claims court. Not to worry, AF does business in many states in the US and finding a registered agent is easy.
Might as well also file a DOT complaint (online form). I don't see DOT enforcing deadlines for responses on carriers these days, but at least you will get higher level attention sooner or later.