The issue when a navigation menu isn’t displayed on the frontend even if you have set definite categories correctly is common for Magento 2.  Thus, you’d better learn how to deal with it even before creating a broad category system in your Magento 2. 

Why is Magento 2 menu not showing and how to fix it?

The issue occurs when you change the Root category: if you change the default category or create new Root category, the Navigation won’t recognize it without additional settings.

Step #1: add a Root Category

• Log in to the Admin Panel;
• Navigate to Product>Category and choose the ‘Add Root Category’ option:

• Give it a name and make sure the triggers above are switched to ‘Yes’.
• Click the ‘Save’ button.

Step #2: create a subcategory of the new Root Category

• choose the just-out category (we have named it: example) and choose the ‘Add Subcategory’ option:

• Make sure the name is given, the triggers are set to ‘Yes’ and the ‘Included on the Nav Menu’ is changed to ‘Yes’;
• Don’t forget to save the subcategory.

Step #3: choose the new Root Category

• Return to the main sidebar, choose Stores>All Stores, select the Main Website Store option and look through the content:

• Choose the Root Category drop-down menu and click the category you’ve just created:

• Click the ‘Save Store’ button.

All the newly-created categories will now appear on the Navigation menu.