The child categories are not displaying in the menu. What might be the issue?
There are several settings that might affect this behavior. First, please check if the root category is included in the menu at Magento Admin Panel → Catalog → Categories → Default Category → Include in Menu.
If a store has several storeviews, it is recommended to check current storeview scope at which the Include in Menu setting is configured.Then, make sure to have the {{child_categories_content}} variable in the content of the main category menu at Mega Menu (For Desktop) → Content:
Switch the Hide Menu Item Content setting to No. You can also find it in the Mega Menu (For Desktop) tab:
And finally, select either vertical or horizontal layout in the Submenu Template field: