Mqtt Sensor is unknown

Configuration
2

You might have to setup the various information in the payload as sensors manually in configuration.yaml. There seems to be several types of data in the payload. The addon may not be setup correctly or just not recognized by Homeassistant. You have an MQTT payload so you can still use it.

mqtt:  
  sensor:  
    - name: "flow temperature"
      device_class: "temperature"
      state_topic: "homeassistant/binary_sensor/wmbusmeters/76406477_status_burst/config"
      value_template: >
        {% if value_json is defined and value_json.id == 76406477 %}
          {{ (value_json.flow_temperature_c)|round(1) }}
        {% else %}
          {{ states('sensor.flow_temperature')|round(1) }}
        {% endif %}
      unit_of_measurement: "°C"

for example

I see someone else has set it up on HA
https://community.home-assistant.io/t/add-on-request-wmbusmeter/228988/13