Access denied on hello world custom module
The issue is with the indentation of the hello_world.routing.yml file. Simply change the indentation of it to:

hello_world_settings:
  path: '/hello/world'
  defaults:
    _controller: '\Drupal\hello_world\Controller\HelloWorldController::myCallbackMethod'
    _title: 'Hello World'
  requirements:
    _permission: 'access content'


And the Access denied issue should be solved. The indentation of YAML file is  important, in Drupal 8. It should be as follow:


path, defaults, and requirements should be indented by 2 spaces
_controller, _title, and _permission should be indented by 4 spaces