Hi,
I am sorry for belated response to this.
If you have modified the header.php file from child theme, you’ll need to add <?php wp_body_open(); ?>
line right after the body tag opening.
So your child theme’s header.php will look like this:
<?php
/**
* The header for our theme.
*
* Displays all of the <head> section and everything up till <div id="content">
*
* @package Sydney
*/
?><!DOCTYPE html>
<html <?php language_attributes(); ?>>
<head>
<meta charset="<?php bloginfo( 'charset' ); ?>">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="profile" href="https://gmpg.org/xfn/11">
<link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>">
<?php wp_head(); ?>
</head>
<body <?php body_class(); ?> <?php sydney_do_schema( 'html' ); ?>>
<?php wp_body_open(); ?>
<?php // Rest of the code ?>
Adding further, you may want to check the entire code of header.php file of newer version of Sydney in this WP SVN repository and some adjustments to your child theme’s accordingly.
Hope this reply helps.
Regards,
Kharis