Spot Fleet Instance could not be stopped

Interruption Behaviour is what AWS will do when your spot instance get interrupted, not what you can do on the instance.

Also, when you use Spot Fleet, the instances are managed by the Fleet. So you cannot stop the instance manually.

Hello,

The AWS Spot Fleet controls the instances lifetime and it automatically manages them on you behalf. Since your fleet includes Spot Instances, Amazon EC2 can attempt to maintain your fleet target capacity as Spot prices change.

An EC2 Fleet request of type maintain or request remains active until it expires or you delete it.

When you delete a fleet of type maintain or request, you can specify whether deletion terminates the instances in that fleet.

Otherwise, the On-Demand Instances run until you terminate them, and the Spot Instances run until they are interrupted or you terminate them.

To get more information about Spot Fleets, please refer to Spot Fleet configuration strategies