为什么页面未找到系统无法在.htaccess文件中工作?

I am using following .htaccess rule to show not-found.php page when user enter wrong url e.g. www.myurl.com/wrong

my .htaccess file :

ErrorDocument 404 /not-found.php
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*)$ $1.php [NC,L]

But somehow it's not working. it's showing me

Not Found

The requested URL /myfolder/mysite/gift-cardsdf was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Apache/2.4.16 (Win32) OpenSSL/1.0.1p PHP/5.5.28 Server at localhost Port 80

How can I fix it ?