Are you sure that Spamassassin (SA) is installed and running on the server?
When you purchased your Plesk license, did you purchase the SA option which would allow you to control some aspects of SA from the control panel?
To check if it is running, you need to login to the server using SSH and give the command:
ps -ax |grep 'spam'
This should show you some lines which would indicate that it is running or not. Please post your results here.
Alternatively, you could give the command:
rpm -qa |grep 'spamassassin'
It should give you 2 lines indicating that the spamassassin and psa-spamassassin are installed, and their versions. Again, please post the results.