As the only service that really uses port 5678 is n8n, I would say the error message is right and you already have another instance of n8n running. If you stop it, all should start totally fine.
Yes, I have successfully started it, but how can I write my own custom node? What shall I do?
Glad to hear that you got it working!
A good starting point for something like that is always the docs. There you can, for example, find the node creation guide where everything should be clearly documented. Here the link:
Additionally, is there also a YouTube video that you can check out:
I have built the programming style environment in the document with the document. And how do I look at my node when I can reach the UI interface?
Sir, are you still there?
There is no Example Node unless you install it, as you can also see in the video here: Learn how to build your own n8n nodes - YouTube
He actually does exactly what you just described: looks for the node, sees it is not there, and then shows what to do to get it properly installed so that it is displayed.
I advise you to look at the video from the very beginning to the end and follow step by step everything he is doing.
I followed the video, but I couldn’t find the HttpBin node and the Example node on the ui page.