Then the template under state in your example goes in the field “State*”… and everything else goes in their matching fields, except unique_id which is not available in the UI.
Compare the code in your first post with that in your screenshot.
Hint: it looks like you’ve omitted the quotation marks around the binary sensor entity_id in the is_state function. This function won’t return a value without them, hence ‘unavailable’. You also need quotes around the state value: 'on'.
You’ve also changed the decimal separator from a point to a comma. Not sure if that is important for how you intend to use this sensor though.
As Chris has pointed out above, you are missing quotes around the entity ID and state args in your is_state() function. Without those quotes Jinja interprets binary_sensor.day_tariff as a variable instead of a string, and you haven’t defined a value for that variable… which will cause an error. IIRC, on will be treated as a boolean, but the state arg expects a string, so that will cause a different error.