why doesnt Drupal recognize my route name?
That code snippet looks very strange, is it really like that in your yml file or is it just wrong in the question here?

There is no route_name key inside the definition, and everything needs to be inside the first key, which is the route name. There are plenty of examples in system.routing.yml. Always try to find actual examples in Drupal core and not some online tutorials when you're not sure, as they might be outdated.

Make sure that you always use two spaces for indentation.

Your example should look like this:

mymodule.admin:
  path: '/admin/structure/mymodule'
  defaults:
    _controller: '\Drupal\mymodule\Controller\QuicktabsController::adminOverview'
    _title: 'mymodule'
  requirements:
    _permission: 'administer mymodule'