You have associated a scaling configuration source with the scaling group, and the scaling group's scaling policy, subnet, and scaling configuration source meet the following requirements. Otherwise, the scaling group cannot be enabled as expected:

Scaling policy	Requirements for the subnet and scaling configuration source
Priority policy	The instance type and image of the scaling configuration source currently associated with the scaling group must be available in the availability zone corresponding to the scaling group’s highest-priority subnet (that is, the first selected subnet).
Balanced policy	The instance type and image of the scaling configuration source currently associated with the scaling group must be available in the availability zones corresponding to all selected subnets of the scaling group.

Procedure

Click the "Enable" button on the right side of the scaling group that you want to enable.