I found the answer to the problem just now:
On the Appearance > Menus page, go to “Screen Options” on the top-right corner. There you’ll find a toggle for “Description.”
I still can’t find the “Description” field.
I did notice in your screenshot that your menu item type is “Custom.”
When I imported the sample content, my menu items were type “Page.” I’ve also tried the “Custom Links” menu type. Neither of them have the “Description” field. But I can’t find the “Custom” type from your example.