Oh, I misunderstood, actually your error comes from the script, not Gladys!
Are you sure you launched the script with the correct parameters? Can you describe exactly what you are doing?
To better understand MQTT in Gladys, I recommend reading this tutorial:
In short, in Gladys the MQTT API is very simple: each sensor has its MQTT topic and listens on that topic. As soon as a new value is sent to that topic, Gladys stores the value. Very simple 
As you said, in the case of this script, this script was coded for Home Assistant, and therefore it does not follow the same topic/message format conventions as Gladys.
So you will need to modify the script to change the topics and the message format to publish
@AlexTrovato Small question, don’t we manage MiFlora in Gladys 4 with the Bluetooth service?