Sorry, I don't know about that node. Are you sure the {{...}} syntax is valid there? If no-one here is able to help then maybe a HA forum would be a better bet.
As far as i am aware The {{..}} is not valid in json input field.
You would need to use function/template/change node to create the json and then feed it in by using dropdown msg.payload. Or use JSONata if available in dropdown.