.htaccess in Subfolder not working

.htaccess in Subfolder not working

I have a url Like
http://localhost/coupon/stores.php?store_slug=url
I want to change it like this
http://localhost/coupon/url
I tried with this code
RewriteEngine on
RewriteBase /coupon/
RewriteCond %{REQUEST_FILENAME} ! -f
RewriteCond %{REQUEST_FILENAME} ! -d
RewriteRule (.*) stores.php?store_slug=$1
but its not working with the above mentioned .htaccess code am getting 500
internal server error. I am using this .htaccess file in sub folder ie
coupon
can anybody check where am doing wrong ?