You do not have a link list with a handle set to popular-themes, or you do and it's empty. Go ahead and create a link list called Popular Themes 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 popular-themes.
Not sure what product you want, but know the theme you would like? Check out all the most popular themes below to see what products you can get by theme.