Your ground speed must be at or above 40 kts besides spoiler extension for the max mode activation
From FCOM DSC-32-30-10 P 7/14
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.