i am assuming that S:\ is a mapped drive. normal culprit for these errors is that the account under which the sql agent service is running does not have that drive mapped or does not have permissions to the drive

You have two options

1) Log in as the service user and make sure you can create files in the s:\ location which the backups are going into.

2) Change the S:\ to the actual UNC path of the backup directory (if you have a permissions issue with S:\ you will still have the permissions error with this)

John