Annotated Registering & Displaying Menus by Joseph Dickson (joseph-dickson.com)

wp_nav_menu( array( 'theme_location' => 'footer-menu' ) ); ❧

Typo? I suspect Joseph may have transposed the ‘footer-menu’ an ‘header-menu’ in these two sections.
I imagine that header-menu belongs in the header.php section.