When you create a scan you can configure it to automatically clean up detected malware. However, in some instances cleanup may require administrator privileges.
Known to apply to the following Sophos product(s) and version(s)
Sophos Anti-Virus for Mac OS X
All supported operating systems
What To Do
Ensure that you are logged in as a user when you run the scan, then if cleanup is required you can carry this out in Quarantine Manager.
Alternatively, if administrator privileges are required, log in as an administrator and re-run the scan.