The SpamAssassin service is always stopped when the Warden Anti-spam and Virus Protection extension is installed:
# systemctl status spamassassin
● spamassassin.service
Loaded: masked (Reason: Unit spamassassin.service is masked.)
Active: inactive (dead)
Warden disables the SpamAssassin service because Amavis executes SpamAssassin subroutines directly, so it is expected that there is no separate process for SpamAssassin. Because of this leaving the SpamAssassin service running would be a waste of memory and server resources. You should not try to enable it.