Could not reliably determine the server's fully qualified domain name (우분투에서 apache설치중 오류)

This is just a friendly warning and not really a problem (as in that something does not work).

If you insert a

ServerName localhost   

in either httpd.conf or apache2.conf in /etc/apache2 and restart apache the notice will disappear.

If you have a name inside /etc/hostname you can also use that name instead of localhost.

And it uses 127.0.1.1 if it is inside your /etc/hosts:

127.0.0.1 localhost
127.0.1.1 myhostname

결론 httpd.conf 나 apache2.conf 파일에 ServerName localhost   써주면 된다.