warning: connect to Milter service inet:127.0.0.1:12768: Connection refused

You have both Plesk milter and OpenDkim listed in non_smtpd_milters, change it to…

milter_default_action = accept
milter_protocol = 6
smtpd_milters = , inet:127.0.0.1:8891, inet:127.0.0.1:12768
non_smtpd_milters = inet:127.0.0.1:8891

Hope it helps.
Kind regards