Getting an error after trying to send notification through Firebase Cloud Functions (Android) - node.js

Question

I am new to Firebase and to nodejs. I am trying to send a notification from one device to another using Firebase Cloud Functions.
This is the node.js code of sending the notification:
var functions = require('firebase-functions');
var admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification = functions.database.ref('/sendNotification/{notificationId}')
.onWrite(event => {
var regToken="fYRweeL8cic:APA91bH6Q_gyKKrLL...";
// Grab the current value of what was written to the Realtime Database.
var eventSnapshot = event.data;
var payload = {
data: {
title: eventSnapshot.child("title").val()
}
};
// Set the message as high priority and have it expire after 24 hours.
var options = {
priority: "high",
timeToLive: 60 * 60 * 24
};
admin.messaging().sendToDevice(regToken,payload,options)
.then(function(response){
console.log("Successfully sent message: ", response);
})
.catch(function(error){
console.log("Error sending message: ", error);
})
})
This is the code of adding the notification to the Realtime Database in order to trigger the function:
public void sendNotification(){
FirebaseDatabase database = FirebaseDatabase.getInstance();
final DatabaseReference myRef = database.getReference("sendNotification");
myRef.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
Toast.makeText(getApplicationContext(),
"sent", Toast.LENGTH_SHORT).show();
Map data = new HashMap();
data.put("title", "this is my title");
data.put("message", "this is the message");
myRef.push().setValue(data);
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
I can see that the function was executed, but with the following error:
The notification appears in the database:
This is how the function appears in the console:
The problem is that the notification is not sent.
I'm getting this: {results:[{error: [Object]}] for some reason.
What can be the cause of this error?
EDIT: (Solution)
As suggested in the comments, I have used this: JSON.stringify(response) to get some more information. This was the response:
{"results":[{"error":{"code":"messaging/registration-token-not-registered","message":"The provided registration token is not registered. A previously valid registration token can be unregistered for a variety of reasons. See the error documentation for more details. Remove this registration token and stop using it to send messages."}}],"canonicalRegistrationTokenCount":0,"failureCount":1,"successCount":0,"multicastId":6051985890611026000}
The response was really clear, the token has changed. I have changed it to a valid token and it worked.

Answer 1

As suggested in the comments, I have used this: JSON.stringify(response) to get some more information. This was the response:
{"results":[{"error":{"code":"messaging/registration-token-not-registered","message":"The provided registration token is not registered. A previously valid registration token can be unregistered for a variety of reasons. See the error documentation for more details. Remove this registration token and stop using it to send messages."}}],"canonicalRegistrationTokenCount":0,"failureCount":1,"successCount":0,"multicastId":6051985890611026000}
The response was really clear, the token has changed. I have changed it to a valid token and it worked.