Play Video on Homepage Only

querySelector() returns null if no matches are found, so wrap the code that depends on imagineVideo in a conditional statement.

var imagineVideo = document.querySelector('.page-id-36 .et_pb_video_box video');
if ( imagineVideo ) {
  imagineVideo.play();
  imagineVideo.loop = true;
  imagineVideo.controls = false;
}

If you only want this code to be loaded on the homepage, you can put it in a separate file and only enqueue it if the homepage is being viewed. For example, you’d place the code above in {your-theme}/js/homepage-video.js. Then you’d enqueue both the main video player script and homepage-video.js:

add_action( 'wp_enqueue_scripts', 'wpse_homepage_video_script' );
function wpse_homepage_video_script() {
    if ( is_front_page() ) {

        // Enqueue the main video player.
        wp_enqueue_script(
            'video-player',
            get_stylesheet_directory_uri() . '/js/video-player.js',
            array(),
            null,
            true
        );

        // Enqueue the JS that controls the video on the homepage.
        wp_enqueue_script(
            'wpse-homepage-video',
            get_stylesheet_directory_uri() . '/js/homepage-video.js',
            array( 'video-player' ),
            null,
            true
        );
    }
}