I’ve successfully connected my Lora node to TTN and now I can receive the node data in node-red. In my arduino code (shared folder), I just send text “{hello}” and try to decode in node-red at the debug node. Attached please find my flow in node-red in my shared folder.
I have also configure the data formatter in TTN such that data bytes can be seem in node-red.
In the meantime, show us what is coming out of the mqtt node by sending it to a debug node. If it is the mqtt node that is showing the error then set it to Auto Detect mode.
If you are a beginner with node red I recommend watching this playlist: Node-RED Essentials. The videos are done by the developers of node-red. They're nice & short and to the point. You will understand a whole lot more in about 1 hour. A small investment for a lot of gain.
Pay particular attention to the section Working with messages.