You can only activate ACC if the vehicle speed is above 20 mph (32 km/h).

When the system is turned on and in the READY state, the Electronic Vehicle Information Center (EVIC) displays “Adaptive Cruise Ready.” When the system is OFF, the EVIC displays “Adaptive Cruise Control Off.”

NOTE:
You cannot enable ACC under the following conditions:
• When you apply the brakes.
• When the parking brake is set.
• When the automatic transmission is in PARK, REVERSE or NEUTRAL.
• When pushing the RES + button without a previously set speed in memory.