Two theme locations for two menus, but only one is showing up

theme-location should be theme_location Check documentation wp_nav_menu

corrected code

wp_nav_menu( array(
    'container' => 'ul',
    'menu_class' => 'mainNav',
    'theme_location' => 'Pages'
) );

wp_nav_menu( array(
    'theme_location' => 'Categories'
) );