You do not have a link list with a handle set to 2015-2017-mustang-v6, or you do and it's empty. Go ahead and create a link list called 2015-2017 Mustang V6 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 2015-2017-mustang-v6.