Hello,
First of all, check which HTML block corresponds to the menu item. Dashboard > HTML blocks. If you want to create new, please note, Megamenu block is created by means of the Extra Menu list element. At the same time, Megamenu block can contain any other static content element such as Product Grid, Images, Texts blocks.
As soon as Megamenu blocks are ready, they may be added to menu items in Appearance > Menu > Select menu > enter menu item > HTML Block dropdown in the Custom fields [for theme] which appear as soon as the menu item has been saved.
You need to set “Full-width” or “Set size” in the Design filed in order to get the option of HTML blocks drop-down.
Best Regards