Undefined variable in functions.php file [closed]

You have made a mistake in the function wpb_related_pages. Your code should look as below:

Updated Code with change needed:

// related pages
function wpb_related_pages() { 
# Global $post variable should be called at the start of the function.
global $post;
$orig_post = $post;
$tags = wp_get_post_tags($post->ID);

Hope this helps!