Unable to link CSS file

First you need to register the style and css using wp_register_script() and wp_register_style() functions

 //registering css
 wp_register_style ( 'mysample', plugins_url ( 'css/MapStyle.css', __FILE__ ) );
 wp_register_script ( 'mysample', plugins_url ( 'js/MapStyle.js', __FILE__ ) );

After this you can call the wp_enqueue_style() functions for loading the css in required page

wp_enqueue_style('mysample');
wp_enqueue_script('mysample');

404 Not Found

Not Found

The requested URL was not found on this server.

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