https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbcluster.html says that PreferredBackupWindow is used if automated backups are enabled using the BackupRetentionPeriod parameter.
It also says that BackupRetentionPeriod Must be a value from 1 to 35.
It is actually possible to disable the automated back-ups? Setting BackupRetentionPeriod to 0 using CloudFormation return the following error: Invalid backup retention period: 0. Retention period must be between 1 and 35.
Unfortunately, you can't disable automated backups on Aurora. Even if you wanted to work around the issue by finding the most recent backup with
aws rds describe-db-cluster-snapshots --db-cluster-identifier=dbname | jq -r .DBClusterSnapshots[].DBClusterSnapshotIdentifier | tail -n1
and then attempting to manually delete the backup with
aws rds delete-db-cluster-snapshot --db-cluster-snapshot-identifier rds:dbname-2021-03-30-04-56
this results in the error
An error occurred (InvalidDBClusterSnapshotStateFault)
when calling the DeleteDBClusterSnapshot operation:
Only manual snapshots may be deleted.