Automation needs to join a text value and a variable when the luminescence changes

You’re not getting the state. You just have an entity_id in there without quotes (to make it a string). See this post about templating types. This means that the template is trying to access a sensor object that doesn’t exist. You need to get the state from the state machine using one of the template functions, like states().

{{ states('sensor.fibaro_system_fgms001_zw5_motion_sensor_luminance') }}