The reason is that there is no response from the server to a request on /. That is something you write in the next exercises. The way it works is more or less like this:
You write a node js script that listens for requests at a specific port
The script filters requests by method and route
You write the code for the response.
In this case, there is a server listening in port 3000, but there is no response to the requests.
To solve all that mess, you need first to learn how to use Node JS.
For anyone who struggled like me and this person completing this lesson with replit, follow the below mentioned steps:
Don’t use the Replit starter boiler link provided in the exercise page. Your submission will fail.
Instead, use the GitHub repo link provided on the same page & create a new Replit project by Importing the boilerplate from the GitHub repo link provided.
Now follow the rest of the exercise as per the steps in the new cloned project, and you should be able to successfully complete the exercise.