firebase deploy error in firebase cloud messaging - firebase

Question

I integrate the chat application. when I log in I add fcm token in the firebase database and when sender and receiver chat I got chat notification on both sides.so I did below step
install node.js
I use node version (v13.11.0) and npm version (6.13.7)
then I use npm install -g firebase-tools (firebase-tools#7.16.1 updated 1 package in 18.481s)
firebase login (Already logged in as)
create a folder and add path then execute firebase init cmd after select function then add my project, select language, install npm ( Firebase initialization complete!)
I add below code in index.js
'use strict'
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification = functions.database.ref('/Notifications/{receiver_id}/{notification_id}')
.onWrite((data, context) =>
{
const receiver_user_id = context.params.receiver_user_id;
const notification_id = context.params.notification_id;
console.log('We have a notification to send to :' , receiver_user_id);
if (!data.after.val())
{
console.log('A notification has been deleted :' , notification_id);
return null;
}
const DeviceToken = admin.database().ref(`/Users/${receiver_user_id}/device_token`).once('value');
return DeviceToken.then(result =>
{
const token_id = result.val();
const payload =
{
notification:
{
title: "New Chat Request",
body: `you have a new Chat Request, Please Check.`,
icon: "default"
}
};
return admin.messaging().sendToDevice(token_id, payload)
.then(response =>
{
console.log('This was a notification feature.');
});
});
});
execute Firebase deploy command I got below error
=== Deploying to 'notification-ad12a8'...
i deploying functions
i functions: ensuring necessary APIs are enabled...
+ functions: all necessary APIs are enabled
i functions: preparing functions directory for uploading...
**Error: An unexpected error has occurred.**
I tried 3 -4 times but not got any solution.please check code please help me..thank you