404 Error in Wordpress Posts & .htaccess Help!

Dear Friends,

I’m trying change rewrite files both in Apache file & Wordpress .htaccess. I’m following below guideline but not help. Website is accessible but not the post and other pages. Why?

https://www.digitalocean.com/community/tutorials/how-to-set-up-mod_rewrite

Thank you,

root@XYZ:~$ sudo apt-get install apache2

[sudo] password for root:

Reading package lists... Done

Building dependency tree

Reading state information... Done

apache2 is already the newest version.

0 upgraded, 0 newly installed, 0 to remove and 69 not upgraded.

root@XYZ:~$ sudo nano /etc/apache2/sites-available/default
root@XYZ:~$ sudo service apache2 restart
 * Restarting web server apache2                                                
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message