AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 172.17.0.2. Set the 'ServerName' directive globally to suppress this message code example

Example: httpd: Could not reliably determine the server's fully qualified domain name, using 192.168.123.156. Set the 'ServerName' directive globally to suppress this message

If you go to:

/etc/apache2/apache2.conf
and insert:

ServerName localhost   
and then restart apache by typing into the terminal:

sudo systemctl reload apache2