Welcome to the forum @henrychanflag
If you want to share some of your flow then you can paste it in here. Make sure you only export the nodes that matter. Here is how to do it:
In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```)
```
code goes here
```
See this post for more details - How to share code or flow json
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.
