If a snapshot is currently in use with an AMI, our policy will not be able to delete the snapshot and will report a failed action against the impacted snapshot. 

If you have a requirement to delete snapshots that are currently associated with an active AMI, please review our ability to use a lambda function within a policy action as this does allow the flexibility to evaluate and deregister an AMI before a snapshot is destroyed within AWS. 

For more information regarding utilizing lambda within the platform, see:  Lambda Functions