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.