To show mega menu layouts, you need to menu first.
From your Shopify admin, go to Online Store -> Themes.
Find the theme that you want to edit, and then click Customize.
In the Header section on the left editor window -> Click + Add block.
Select block type is "Mega Menu".
Navigation item
Enter the name of the collection for which you want to apply the mega menu layout into the text box.
Section width
Select the section width from the dropdown menu:
Full width
Page width
Custom width
Customize width
Determines the width of the section when you choose the 'Custom width' option. It can range from 1000px to 1800px, increasing by 50px increments
Image
Set up an image
Heading
Add text to display as a block heading (Leave the field blank to hide the heading)
URL
The link that you want customers to navigate to if they click the image
Image overlay opacity on hover
Determines the image overlay opacity of the block. Can be from 0% to 100%, in 10% arithmetic increments
Background
Set the background color for this block