I haven't installed any extension like admin tools. Also , I've already tried to change htaccess rules but I didn't gain any result. I would be appreciate if you take a look at my htaccess:

##
# @package Joomla
# @copyright Copyright (C) 2005 - 2016 Open Source Matters. All rights reserved.
# @license GNU General Public License version 2 or later; see LICENSE.txt
##

##
# READ THIS COMPLETELY IF YOU CHOOSE TO USE THIS FILE!
#
# The line just below this section: 'Options +FollowSymLinks' may cause problems
# with some server configurations. It is required for use of mod_rewrite, but may already
# be set by your server administrator in a way that disallows changing it in
# your .htaccess file. If using it causes your server to error out, comment it out (add # to
# beginning of line), reload your site in your browser and test your sef url's. If they work,
# it has been set by your server administrator and you do not need it set here.
##

## No directory listings
IndexIgnore *

## Can be commented out if causes errors, see notes above.
Options +FollowSymlinks
Options -Indexes

## Mod_rewrite in use.

RewriteEngine On

## Begin - Rewrite rules to block out some common exploits.
# If you experience problems on your site block out the operations listed below
# This attempts to block the most common type of exploit `attempts` to Joomla!
#
# Block out any script trying to base64_encode data within the URL.
RewriteCond %{QUERY_STRING} base64_encode[^(]*\([^)]*\) [OR]
# Block out any script that includes a <script> tag in URL.
RewriteCond %{QUERY_STRING} (<|%3C)([^s]*s)+cript.*(>|%3E) [NC,OR]
# Block out any script trying to set a PHP GLOBALS variable via URL.
RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]
# Block out any script trying to modify a _REQUEST variable via URL.
RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2})
# Return 403 Forbidden header and show the content of the root homepage
RewriteRule .* index.php [F]
#
## End - Rewrite rules to block out some common exploits.

## Begin - Custom redirects
#
# If you need to redirect some pages, or set a canonical non-www to
# www redirect (or vice versa), place that code here. Ensure those
# redirects use the correct RewriteRule syntax and the [R=301,L] flags.
#
## End - Custom redirects

##
# Uncomment following line if your webserver's URL
# is not directly related to physical file paths.
# Update Your Joomla! Directory (just / for root).
##

# RewriteBase /

## Begin - Joomla! core SEF Section.
#
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
#
# If the requested path and file is not /index.php and the request
# has not already been internally rewritten to the index.php script
RewriteCond %{REQUEST_URI} !^/index\.php

# and the requested path and file doesn't directly match a physical file
RewriteCond %{REQUEST_FILENAME} !-f
# and the requested path and file doesn't directly match a physical folder
RewriteCond %{REQUEST_FILENAME} !-d
# internally rewrite the request to the index.php script
RewriteRule .* index.php [L]
#
## End - Joomla! core SEF Section.

I should add this, I have exactly the same htaccess ( and extensions) on the local host but there isn't any problem! The only difference is that I have joomla 2.5 there and a 3.4 version here.

Thanks again.

Another clue: When chrome inspect error console is open and I click on submit I see this:
"POST vrnegar.com/index.php 403 (Forbidden) jquery.min.js:5"

imgur.com/ZcDfquD

what does it mean?

It means that the AJAX call was blocked, can you revert to Joomla!'s default .htaccess file temporarily?

Replaced the htaccess file with joomla default htaccess, which was in its installation pack but nothing has changed. Still same error is shown and submit button doesn't work. :(
I think its something about joomla version and jquery files?isnt it?

Can you enable the comments for the public so I can test them please?

Thank you. The comment form is now available for public at the end of the page:
vrnegar.com/software/item/3-2

And if you need more access and information I would be pleased and grateful to give them to you.

Something is still blocking all POST requests.

<p>You don't have permission to access /index.php
on this server.<br />

Since you have already reverted to the default .htaccess file, I would suggest that you let your hosting provider about this issue, since there may be custom security rules.

Thanks krikor.I called service provider and they told me they have blocked the comment service because of security of the site! They asked me to enable google recaptcha and then they solved the problem.

Thanks again.

Nice to hear that the issue has been resolved Hesam :)