Introduction:
After having done a dist-upgrade fo Jessie to Stretch OpenDKIM didn’t start any more.
After research I found the answer which worked for me in this site:
https://serverfault.com/questions/847435/cant-change-opendkim-socket-in-debian-stretch-in-etc-default-opendkim

INFO:
I’m using the ‘inet’ socket for the communication between Postfix and OpenDKIM at port 12345.
eg. My config in of OpenDKIM in Postfix:
milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:localhost:12345
non_smtpd_milters = inet:localhost:12345

Solution found in the above web site:
/lib/opendkim/opendkim.service.generate
systemctl daemon-reload
service opendkim restart

Note: There are other solutions for the ones that use other kind of communication sockets for the communication between Postfix and OpenDKIM found in the same above site.

Solution: Regenerate the proper