If you didn’t add the ‘www’ prefix at the time of setting up the domain, even adding it back in later doesn’t seem to work. This fault seems to be present within all current versions of Plesk up to my current version v9.2
To work around this problem, firstly – turn on the ‘www’ prefix again within the Plesk control panel.
Domains > yourdomain.com > Modify > WWW Prefix – Tick this box
The www prefix will still not work… to correct this, we need to re-build the Apache config after turning it back on:
/usr/local/psa/admin/sbin/websrvmng -v -a