admin.messaging().send(message) not working in firebase cloud mesaging node js code example

Example: node js send fcm

var FCM = require('fcm-node')
    var serverKey = require('path/to/privatekey.json') //put the generated private key path here            
    var fcm = new FCM(serverKey)     
    var message = { //this may vary according to the message type (single recipient, multicast, topic, et cetera)        
      to: 'registration_token',         
      collapse_key: 'your_collapse_key',                
      notification: {            
        title: 'Title of your push notification',             
        body: 'Body of your push notification'         
      },                
      data: {  //you can send only notification or only data(or include both)            
        my_key: 'my value',            
        my_another_key: 'my another value'        
      }    
    }        
    fcm.send(message, function(err, response){        
      if (err) {            
        console.log("Something has gone wrong!")        
      } else {            
        console.log("Successfully sent with response: ", response)        
      }    
    })