Realted product section sidebar menu option not working for my shopify site

We have purchased the theme https://themeforest.net/item/besto-the-electronics-fashion-multipurpose-ecommerce-shopif


y-theme/28359093 for our shopify website the problem i am facing now is if i go to product page using customize theme option and click on product page at that time its not displaying me sidebar menu option to edit related product section, please find attachment for the same how can i edit those related product list