Prestege Theme Problem

Hello Guys ,
I have problem with wordpress theme its call ( PRESTEGE ) The problem is when i start to build main menu , i start to create the pages and give it the template that i want , like gallery page , if any body know this theme its need to nextgen gallery , i have done of downloading the plugin and installed but its my first time to use this plugin ” im wordpress beginner ” so after publishing the page i try to click to view the page, its give me
Not Found

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The requested URL /website/gallery/ was not found on this server.

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

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Please guys i need help