Your Item is defined using MQTT 1.x version syntax. If you have the MQTT 2.5 binding installed, you need to create Things and Channels to subscribe to the topic and then link your Item to that Channel. You are doing a mix of all sorts of things which don’t make much sense.
Also, please use How to use code fences.
See the MQTT Docs, and there are tons of examples and related threads you can use if you search the forum for MQTT 2.5.