Call to undefined function get_header()

Hello,

i am using this code to going to page in my directory
theme there is ch folder

<li>
    <a href="<?php echo get_stylesheet_directory_uri(); ?>/ch/bn1.php">
        <img src="<?php echo get_stylesheet_directory_uri(); ?>/img/bn1.png">
    </a>
</li>

but going to page url wp-content/themes/live/ch/bn1.php showing this message

Fatal error: Call to undefined function get_header() in C:\wamp\www\blog\wp-content\themes\live stream\ch\bn1.php on line 1