Besides always having the complet actual blueprint is really a good idea.
If you are using the Version from your first post, the indentation is not correct.
Try this:
blueprint:
name: Tado open window detection
description: turns the Tado off when open window is detected
domain: automation
input:
open_window_sensor:
name: Open Window Sensor
selector:
entity:
domain: sensor
target_climate:
name: Climate Device
selector:
entity:
domain: climate
trigger:
platform: state
entity_id: !input open_window_sensor
from: 'True'
to: 'False'
action:
entity_id: !input target_climate
type: set_hvac_mode
hvac_mode: auto