You need to show hidden files by clicking on the 3 vertical dots and then open the .replit file and change index.js to server.js, because the app uses server.js as the entry point.
Once you fix that and click Run, you will get an error message that I believe you can easily fix.
I do not see where you defined a variable in your code named MONGO_URI. That is why you are getting the error. Look carefully at the example in the instructions on referencing the environment variable named MONGO_URI.
In the previous curriculum section, do you not remember how to reference an environment variable? If not, I would review the previous app created, to see how you did it there.
The problem is not in the Secrets section. You are doing that correctly. The problem is how you are trying to reference that environment variable in the code.
one of my problems is that i often mix to many program-styles and languages in my mind. and i usually use jetbrains - tools its nearly like a auto-correction-tool