A few years ago, we published a very popular post called "How to Create Dropdown Menus in Drupal".

That post covers many of the basic points that have not changed in Drupal 8, including these:

• Many themes don't have dropdown menus built-in. That includes Drupal's core themes, such as Bartik.
• It is best to choose a theme that does already have dropdowns available.
• You need to go to Structure > Block layout and make sure your menu is placed in the correct block region.
• You need to go to Structure > Menus and make sure your menus links are indented.