npm WARN enoent ENOENT: no such file or directory, open 'C:\Users\Nuwanst\package.json'
Solution 1:
If you already have package-lock.json file just delete it and try again.
Solution 2:
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.
Let me know if this works.