function set_head_keywords() {
$id = get_the_ID();
if (!$id) return;
$meta = get_post_meta( $id, '_yoast_wpseo_focuskw', true );
echo '<meta name="keywords" content="'.$meta.'" />';
}
add_action( 'wp_head', 'set_head_keywords' );
copy paste this code in your function.php and it will work fine