You’re on openHAB 3, which doesn’t have an MQTT Broker. You need to install one separately, such as Mosquitto.
Once that’s done, point zigbee2mqtt to your MQTT broker by editing its configuration.yaml.
Once that’s done, connect openHAB to the MQTT broker via a Broker Thing..
Once that’s done, create a Generic MQTT Thing for your Aqara device. This tutorial goes into detail on a Generic MQTT Thing.