Firebase function deploy error cannot find module firebase-admin

I'm getting this error when I'm trying to deploy a function to firebase. I've used this tutorial. https://firebase.google.com/docs/functions/get-started

My package.json is this.

    {
  "name": "functions",
  "description": "Cloud Functions for Firebase",
  "dependencies": {
    "firebase-admin": "^5.4.2",
    "firebase-functions": "^0.7.1"
  },
  "private": true
}

And my index.js is this

const functions = require('firebase-functions');
// The Firebase Admin SDK to access the Firebase Realtime Database. 
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

// // Create and Deploy Your First Cloud Functions
// // https://firebase.google.com/docs/functions/write-firebase-functions
//
exports.helloWorld = functions.https.onRequest((request, response) => {
 response.send("Hello from Firebase!");
});
// Take the text parameter passed to this HTTP endpoint and insert it into the
// Realtime Database under the path /messages/:pushId/original
exports.addMessage = functions.https.onRequest((req, res) => {
  // Grab the text parameter.
  const original = req.query.text;
  // Push the new message into the Realtime Database using the Firebase Admin SDK.
  admin.database().ref('/messages').push({original: original}).then(snapshot => {
    // Redirect with 303 SEE OTHER to the URL of the pushed object in the Firebase console.
    res.redirect(303, snapshot.ref);
  });
});

Can you tell me whats wrong in the code? Thanks in advance.