Why codeIgniter show the requested URL was not found on this server?

First of all enable mod_rewrite then restart apache2 using following commands:

sudo a2enmod rewritesudo service apache2 restart

then open apache conf file using command:

sudo gedit /etc/apache2/apache2.conf

uncomment below line if it is commented

AccessFileName .htaccess

then find these lines in apache2.conf

<Directory /var/www/>Options Indexes FollowSymLinksAllowOverride NoneRequire all granted

here change the line AllowOverride None to AllowOverride All and save this file.

Now, your issue is resolved.

First thing that you should know the debian is a Linux base server. So Its not like windows servers(XAMPP and WAMP).

So first of all consider about Letters. It means debian Sever is Case-Sensitive.

So check your naming conversation as well

Check these

1. Controller class name class Admin extends CI_Controller
2. Controller file name should be admin.php
3. Check in routes.php. In routes $route['default_controller'] = "admin";

Codelgniter URL example :

example.com/class/function/id/ 

So check your class name correct or not, And call url instead of Capital A - Admin to admin.

<?php (defined('BASEPATH')) OR exit('No direct script access allowed');class Admin extends CI_Controller {}  

Note: if you use codelgniter-3 version file name also must Should capital A. I mean Admin.php