Welcome to the forum @Samira05
That probably means that payload[i].Data.split is not a string. Since this question is unrelated to the original question of this thread please start a new thread for it.
First though, if you are a beginner to node-red then 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.
Also read carefully the node red docs page Working with Messages which you should find helpful.
If you still need help then start a new thread and there show us what you are feeding into the node generating the error and what is in that node. There is probably no need to post the flow. However, for the future, if you do need to post a flow (or code) then see this post for how to do it - How to share code or flow json
