You should not try to start the SpamAssassin service. Warden disables (masks) the SpamAssassin service as Amavis does not use the SpamAssassin daemon directly so starting the service would be a waste of memory and server resources.
If you accidentally enabled the old legacy spam filter settings you can disable them again using the commands below:
systemctl unmask spamassassin
/usr/local/psa/bin/spamassassin --update-server -status false
/usr/local/psa/bin/spamassassin --update-server -status false -personal-conf false
/usr/local/psa/admin/sbin/spammng --disable
systemctl mask spamassassin
