Unable to get a value in a RESTful sensor

Configuration
2

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Since you want to fetch the data from the source, you need to use the GET method instead of the POST method.
And then you have to define a list of the required json attributes.
See this example:

Try this:

sensor:
  - platform: rest
    name: Airzone Guestroom status
    resource: http://192.168.1.242:3000/api/v1/hvac
    method: GET
    json_attributes:
      - systemID
      - zoneID
      - name
      - on
      - maxTemp
      - minTemp
      - setpoint
      - roomTemp
      - mode
      - coldStages
      - coldStage
      - heatStages
      - heatStage
      - humidity
      - units
      - errors
      - air_demand
      - floor_demand

This should provide you with a sensor delivering all the required data as attributes.
You can get specific attributes from that sensor with template sensors like this:

sensor:
  - platform: template
    sensors:
      airzone_systemid:
        friendly_name: "Airzone Guestroom System ID"
        value_template: "{{ state_attr('sensor.airzone_guestroom_status', 'systemID') }}"
      airzone_zoneid:
        friendly_name: "Airzone Guestroom Zone ID"
        value_template: "{{ state_attr('sensor.airzone_guestroom_status', 'zoneID') }}"