The SpamAssassin service is always in a stopped status when the Plesk Email Security extension installed on the server:
# service spamassassin status
● spamassassin.service - Perl-based spam filter using text analysis
Loaded: loaded (/lib/systemd/system/spamassassin.service; disabled; vendor preset: enabled)
Active: inactive (dead)
The Amavis mail filter is installed within the installation of the Plesk Email Security extension.
The Amavis and SpamAssassin are written in Perl, and Amavis executes SpamAssassin subroutines directly, so it is expected that there is no separate process for SpamAssassin.
This is expected behavior, no additional actions are required.