I just figure it out. The problem was because of the root path in file 000-default.conf at /etc/apache2/sites-enabled directory.
If we lunch this file in terminal we'll see the following result:
<VirtualHost *:80>
...
#ServerName www.example.com
ServerAdmin webmaster@localhost
DocumentRoot /var/www/html
# Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
# error, crit, alert, emerg.
# It is also possible to configure the loglevel for particular
# modules, e.g.
#LogLevel info ssl:warn
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
...
</VirtualHost>
As you can see in line DocumentRoot /var/www/html the DocumentRoot has been set to /var/www/html which it should be set on following address :
/var/www/itop
and in file :
<VirtualHost *:80>
...
#ServerName www.example.com
ServerAdmin webmaster@localhost
DocumentRoot /var/www/itop
# Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
# error, crit, alert, emerg.
# It is also possible to configure the loglevel for particular
# modules, e.g.
#LogLevel info ssl:warn
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
...
</VirtualHost>