Max is not arm-able in the air. It's a T/O only brake for RTOs. Just press Low or Med and it should illuminate to indicate armed. The trigger for auto brake is the ground spoilers and the trigger for the ground spoilers is weight on wheels with thrust levers at idle if spoilers armed or at least one reverser open if spoilers not armed.
From FCOM:
SYSTEM ACTIVATION Automatic braking is activated: ‐ at the command for ground spoilers extension (Refer to DSC-27-10-20-C Speed Brakes and Ground Spoilers - Speed Brake Control), for LO and MED mode. ‐ at the command for ground spoilers extension and the aircraft speed is above 40 kt, for MAX mode. Therefore, if the aircraft makes an acceleration stop and begins to decelerate when its speed is under 72 kt, the automatic braking will not activate because the ground spoilers will not extend. For autobrake to activate, at least two SEC's must be operative.