Can't add a php element

I want to add the code on this website: https://presscustomizr.com/snippet/adding-widget-area-home/

add_filter( 'tc_default_widgets' , 'add_featured_page_widget' );
function add_featured_page_widget( $defaults ) {
	$defaults['fp_widgets'] = array(
                    'name'                 => __( 'Featured Pages Widget' , 'customizr' ),
                    'description'          => __( 'Above the featured pages area on home' , 'customizr' )
	);
    return $defaults;
}

add_action('__before_fp' , 'display_my_fp_widget');
function display_my_fp_widget() {
    dynamic_sidebar('fp_widgets');
}

to the functions.php file, but my website doesn’t work when I save it: “Fatal error: Call to undefined function add_filter() in /home/boredom3/public_html/wp-includes/functions.php on line 4999”.

Anyone?