Have you created a package.json file? Maybe run this command first again.

C:\Users\Nuwanst\Documents\NodeJS\3.chat>npm init

It creates a package.json file in your folder.

Then run,

C:\Users\Nuwanst\Documents\NodeJS\3.chat>npm install socket.io --save

The --save ensures your module is saved as a dependency in your package.json file.