Didn't receive real-time developer notifications

Question

I am not receiving real-time developer notifications from Google play store. I have followed following steps:

1. Set up GOOGLE_APPLICATION_CREDENTIALS

2. Set up Listener for messages. Code for message receiver

   public List<ReceivedMessage> recieveAndroidNotification() throws IOException {
   
       String subscriptionId = "my-project-id";
       String projectId = "my-subscription-id";
       SubscriberStubSettings subscriberStubSettings = SubscriberStubSettings.newBuilder().build();
       try (SubscriberStub subscriber = GrpcSubscriberStub.create(subscriberStubSettings)) {
           // String projectId = "my-project-id";
           // String subscriptionId = "my-subscription-id";
           int numOfMessages = 10; // max number of messages to be pulled
           String subscriptionName = ProjectSubscriptionName.format(projectId, subscriptionId);
           PullRequest pullRequest = PullRequest.newBuilder().setMaxMessages(numOfMessages).setReturnImmediately(false) // return immediately if messages are not available
                   .setSubscription(subscriptionName).build();
   
           // use pullCallable().futureCall to asynchronously perform this operation
           PullResponse pullResponse = subscriber.pullCallable().call(pullRequest);
           List<String> ackIds = new ArrayList<>();
           for (ReceivedMessage message : pullResponse.getReceivedMessagesList()) {
               // handle received message
               // ...
               System.out.println("Message Recived");
               ackIds.add(message.getAckId());
           }
           // acknowledge received messages
           AcknowledgeRequest acknowledgeRequest = AcknowledgeRequest.newBuilder().setSubscription(subscriptionName).addAllAckIds(ackIds)
                   .build();
           // use acknowledgeCallable().futureCall to asynchronously perform this operation
           subscriber.acknowledgeCallable().call(acknowledgeRequest);
           return pullResponse.getReceivedMessagesList();
       }
   }
   

3. Publishes a test notification to the Cloud Pub/Sub topic, thereby verifying the setup and configuration of that topic prior to real-time notifications being received. (Haven't received notification)

4. Run code manually then it send error:

   WARN [2018-06-06 06:37:02,479] com.google.auth.oauth2.ComputeEngineCredentials: Failed to detect whether we are running on Google Compute Engine.

Am I missing something?