Template variable error: 'value_json' is undefined - mqtt

[TheLordVader (Anders)]

Tried this without success:

How to purge the retained MQTT messages?

You purge a retained message by publishing an empty string to the topic as a retained message. It’s as simple as that. You can do it directly from Developer tools > Services. In the following screenshot, we are publishing an empty string (represented by two consecutive single quotes) to the topic your/topic/goes/here/ as a retained message (retain: true). [Screenshot from 2021-01-13 21-31-39]