www prefix is not available for Windows Domains Print

This is caused because during domain creation the www prefix checkbox was not selected and the domain has been created without it. To fix that you need to login to Plesk.

Go to domains->enter the domain name that has the problem->select the checkbox in front of the domain name->click the Group Operations (v8.6) or Modify (v9.x) icon above the domain list

Scroll down the list and select Swtich on for the WWW prefix.