I think the hint could be worded a little better. It should say “Your main element should come after your header element” or perhaps “Your opening main tag should come after the header element”.
Remember, a header element consists of both an opening <header> tag and a closing </header> tag. So when the instructions say “Add a main element below the existing header element” that means to add the main element after the entire header element, which includes both the opening and closing tag.