Turn light off after 20 seconds

I’m trying to use a script I found here: https://github.com/CCOSTAN/Home-AssistantConfig/blob/master/automation/System/watchdog_light.yaml but it’s not working.

This is my automation:

- alias: Automated Switch WatchDog!
  trigger:
    - platform: state
      entity_id: 
      - switch.hall_main_light_switch_8_0
      state: 'on'
      for: '00:00:20'

  action:
    - service: switch.turn_off
      data_template:
        entity_id: "{{ trigger.entity_id }}"

And I get this error on the console:

Traceback (most recent call last):
  File "/usr/src/app/homeassistant/helpers/template.py", line 99, in async_render
    return self._compiled.render(kwargs).strip()
  File "/usr/local/lib/python3.5/site-packages/Jinja2-2.9.6-py3.5.egg/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/usr/local/lib/python3.5/site-packages/Jinja2-2.9.6-py3.5.egg/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.5/site-packages/Jinja2-2.9.6-py3.5.egg/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "<template>", line 1, in top-level template code
  File "/usr/local/lib/python3.5/site-packages/Jinja2-2.9.6-py3.5.egg/jinja2/sandbox.py", line 385, in getattr
    value = getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'trigger' is undefined

  During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "uvloop/future.pyx", line 372, in uvloop.loop.BaseTask._fast_step (uvloop/loop.c:112669)
  File "/usr/src/app/homeassistant/components/automation/__init__.py", line 326, in async_trigger
    yield from self._async_action(self.entity_id, variables)
  File "/usr/src/app/homeassistant/components/automation/__init__.py", line 407, in action
    yield from script_obj.async_run(variables)
  File "/usr/src/app/homeassistant/helpers/script.py", line 151, in async_run
    yield from self._async_call_service(action, variables)
  File "/usr/src/app/homeassistant/helpers/script.py", line 181, in _async_call_service
    self.hass, action, True, variables, validate_config=False)
  File "/usr/src/app/homeassistant/helpers/service.py", line 74, in async_call_from_config
    config[CONF_SERVICE_DATA_TEMPLATE]))
  File "/usr/src/app/homeassistant/helpers/service.py", line 70, in _data_template_creator
    for key, item in value.items()}
  File "/usr/src/app/homeassistant/helpers/service.py", line 70, in <dictcomp>
    for key, item in value.items()}
  File "/usr/src/app/homeassistant/helpers/service.py", line 72, in _data_template_creator
    return value.async_render(variables)
  File "/usr/src/app/homeassistant/helpers/template.py", line 101, in async_render
    raise TemplateError(err)
homeassistant.exceptions.TemplateError: UndefinedError: 'trigger' is undefined

Everything looks right to me - any ideas what might be wrong?

indentation looks off here:

  trigger:
    - platform: state
      entity_id: 
      - switch.hall_main_light_switch_8_0

Indent the - switch.hall_main... with two more spaces and see if it helps.