sitemap html (without W3T cache or similar plugin)

When i submit sitemap, google webmastertools gives out

Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.

I am not using any cache plugin, instead I have the htacess file as below,
may Iknow any way to fix the html issues? Thank you.

# Begin cache control #
ExpiresActive on
ExpiresDefault "now plus 1 month"
ExpiresByType text/html "now plus 1 month"
ExpiresByType application/javascript "access plus 1 year"
<FilesMatch "\.(css|png|bmp|ico|htm|gff|html|js|jpg|jpeg|gif|gcf|x-icon|woff|eot|svg|svgz)$">
FileETag MTime Size
ExpiresDefault "now plus 1 month"
</FilesMatch>
# End cache control #

# 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

# WordPress SEO - XML Sitemap Rewrite Fix
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^sitemap_index\.xml$ /index.php?sitemap=1 [L]
RewriteRule ^([^/]+)?-sitemap([0-9]+)?\.xml$ /index.php?sitemap=$1&sitemap_n=$2 [L]
</IfModule>
# END WordPress SEO - XML Sitemap Rewrite Fix

https://wordpress.org/plugins/wordpress-seo/