You do not have a link list with a handle set to 1999-2006-silverado-1500, or you do and it's empty. Go ahead and create a link list called 1999-2006 Silverado 1500 on your Navigation page and populate that link list with links that point to collections, to see collections listed here. Make sure the link list also has a handle set to 1999-2006-silverado-1500.