XML Sitemaps not being generated

The XML Sitempas is not being generated. When I click on domain.com/sitemap_index.xml It redirects me to index page.

When I want to uncheck the “Check this box to enable XML sitemap functionality.” It will auto recheck back.

.htaccess

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

Plugins I Use
Akismet
BackupBuddy
Google Analytics for WordPress
PHP MyVideoBlog
WordPress SEO
WP-PageNavi
WP-PostRatings
WP-PostViews