"Preview Changes" button cannot work

You may need to use the plugin then if you can't change one of the names.

I have found the solution to the problem of display of directory listing over the permalink.
I wanted a permalink to be displayed but because I have a physical folder structure same as my permalink, the wordpress display my directory structure instead of my permalink.

The solution is to set a security in the file ".htaccess" under the website root directory.
Add in the following lines,

#Prevent wordpress from listing the directory's files.
Options All -Indexes

Wordpress will not list out the files under the directory even if index.html is not available.
This is a security measures to prevent hackers from navigating your website's directories.
With this in place, Wordpress will load my permalink instead of the directory now.

Awesome! Thanks for sharing the solution! :)