Link to a custom entity settings page does not appear at /admin/structure
Please revise your file:  


  yourmodule.links.menu.yml.


and verify if exists the parameter


  parent


A complete example

  yourmodule.admin_content:
  title: Content
  description: 'Find and manage content.'
  route_name: system.admin_content
  parent: system.admin_structure
  weight: -10