apache2: Could not reliably determine the server's fully qualified domain name, using code example

Example: mac httpd: Could not reliably determine the server's fully qualified domain name, using

Uncomment this line in httpd.conf

ServerName localhost

The problem is that on macOS it is the wrong file (not /private/etc/httpd.conf).

To find the right location of your httpd.conf Apache configuration file, run:

apachectl -t -D DUMP_INCLUDES
then double check whether ServerName is uncommented and set to localhost.