ERROR "mail transport unavailable" after integrating SpamAssassin with Postfix on CentOS7

Question

I am using Postfix 2.10.1, Spamassassin 5.16.3 on CentOS 7.

PROBLEM: Postfix stopped sending and receiving mail.

/var/log/maillog displays the two lines:

Aug 3 18:56:15 myServer postfix/error[15579]: C1166215DA: to=, relay=none, delay=5.2, delays=5.2/0.01/0/0.01, dsn=4.3.0, status=deferred (mail transport unavailable)

and

Aug 3 18:52:19 myServer postfix/pipe[15124]: fatal: get_service_attr: unknown username: spamd

WHAT CAUSED THE PROBLEM

On Plesk onyx Tools & Settings > Spam Filter: The following 2 are ticked

Switch on server-wide SpamAssassin spam filtering

Apply individual settings to spam filtering

And the following values were set:

The maximum number of worker spamd processes to run = 1

The score that a message must receive to qualify as spam = 7

Then I followed the steps at this link:
https://janikarhunen.fi/tackle-spam-with-spamassassin-on-centos-7-and-postfix

1) added the following to /etc/mail/spamassassin/local.cf :

groupadd spamd

useradd -g spamd -s /bin/false -d /var/log/spamassassin spamd

chown spamd:spamd /var/log/spamassassin

NOTE: I couldn't find "spamassassin" folder in /var/log/ !

2) restarted spamassassin service: systemctl start spamassassin

3) Update the spam rules by running: sa-update

4) edited /etc/postfix/master.cf

smtp inet n - n - - smtpd

replaced with

smtp inet n - n - - smtpd -o content_filter=spamassassin

and added this line at the end of the file:

spamassassin unix - n n - - pipe flags=R user=spamd argv=/usr/bin/spamc -e /usr/sbin/sendmail -oi -f ${sender} ${recipient}

5) restarted Postfix : systemctl restart postfix

Please note that when changes in (4) were reversed, mailing started to work normally.

Please help.