ERROR: (gcloud.app.deploy) Error Response: [9] Application startup error:

In order to run the example you show, please follow the instructions in the docs:

1. Download the credentials JSON file and create an environment variable that points to this file:

export GOOGLE_APPLICATION_CREDENTIALS="/path/to/your/credential-file.json"

2. Copy the code in, i.e., sample.js file and run node sample.js.

On the other hand, if you want to deploy a nodejs app, you'll need express.js or any other framework.

I did the following with the help of express.js:

I wrapped your code in a function called saveEntity(datastore)

Then I added express.js to the app like in the sample nodejs app and from there I called saveEntity(datastore):

app.get('/', (req, res) => {
    saveEntity(datastore);
    res.status(200).send("Entity saved").end();
}); 

// [START listen]
const PORT = process.env.PORT || 8080;
app.listen(process.env.PORT || 8080, () => {
  console.log(`App listening on port ${PORT}`);
  console.log('Press Ctrl+C to quit.');
});
// [END listen]
// [END app]

module.exports = app;

The complete result is this:

// Imports the Google Cloud client library
const Datastore = require('@google-cloud/datastore');
//Needed to create the node app
const express = require('express')

const app = express();

// Your Google Cloud Platform project ID
const projectId = 'my-project-id';

// Creates a client
const datastore = new Datastore({
    projectId: projectId,
});

function saveEntity(datastore){
    // The kind for the new entity
    const kind = 'JulyTask';
    // The name/ID for the new entity
    const name = 'sampletask1';
    // The Cloud Datastore key for the new entity
    const taskKey = datastore.key([kind, name]);

    // Creates entity data
    const task = {
        name: 'Learn something',
        status: 'In progress',
        description: 'Friday 6 July'
    }

    //With the data and the key, create the entity
    const entity = {
        key: taskKey,
        data: task
    }

    // Saves the entity
    datastore
        .upsert(entity)
        .then(() => {
            console.log('Saved:\n' + JSON.stringify(entity));
            return true;
        })
        .catch(err => {
            console.error('ERROR:', err);
            return false;
        });
}

app.get('/', (req, res) => {
    saveEntity(datastore);
    res.status(200).send("Entity saved").end();
}); 

// [START listen]
const PORT = process.env.PORT || 8080;
app.listen(process.env.PORT || 8080, () => {
  console.log(`App listening on port ${PORT}`);
  console.log('Press Ctrl+C to quit.');
});
// [END listen]
// [END app]

module.exports = app;

The package.json:

{
    "name": "first-application",
    "version": "1.0.0",
    "description": "First program using cloud datastore",
    "main": "app.js",
    "scripts": {
        "start": "node app.js",
        "deploy": "gcloud app deploy",
        "test": "echo \"Error: no test specified\" && exit 1"
    },
    "author": "Ragav",
    "license": "ISC",
    "dependencies": {
        "@google-cloud/datastore": "^1.4.1",
        "express": "^4.16.3"
    }
}

Also you should know that this is deprecated:

runtime: nodejs
vm: true

You could do this instead:

runtime: nodejs
env: flex
service: my-service-name

I had the same problem. At the loggin it also said:

ERROR: build step 0

I solved it deleting two of the buckets on the project I was trying to deploy. Go to https://console.cloud.google.com/storage/ and delete the buckets that start with:

stagging....
artifact...

After doing this you can deploy again

I am not sure what was the exact cause of the problem as the error is not clear nor the documentation.

My guess is that there is a problem with storage limit but as I said I can't be sure.