RESOLUTION
If there is no "Spam Filter" button on the mailname page, then it means that SpamAssassin per users settings are disabled on the server wide level at "Server->Spam Filter" page. See if "Allow per-user preference" checkbox is enabled.
If the "Spam Filter" button is shown, but it is greyed and there is no way to manage SpamAssassin settings for the mailname, make sure that 'spamassasin' and 'psa-spamassassin' packages are installed in the system and your Plesk license allows to use Spamassassin.