Hello Abhay,
Thank you so much for using our theme SuperMagPro. SuperMagPro is specially designed for magazine site, due to that, as per the design concept, you can only create mega menu from category/subcategory. If you want to make the mega menu from pages or from other topic, you need to customize the theme or you need to find the suitable plugin for it. Please find the suitable plugin or customize the theme as your need.
Warm Regards,
Acme Supports