You may find that actions to delete an EC2 Snapshot fail with error message - "The snapshot is managed by the AWS Backup Service" when viewing the action under Dashboard -> Notifications -> Actions.
This is an expected error when you attempt to delete an EC2 Snapshots managed via the AWS Backup Service, as per Delete an Amazon EBS snapshot :
You can't delete a snapshot that is managed by the AWS Backup service using Amazon EC2. Instead, use AWS Backup to delete the corresponding recovery points in the backup vault. For more information, see Deleting backups in the AWS Backup Developer Guide.
Please delete the snapshot by removing the corresponding recovery point as per the above.