cloud build trigger in app engine -Nodejs runtime error - node.js

Question

WARNING: Your package.json does not specify a supported Node.js version. Please pin your application to a major version of the Node.js runtime.
Application detection failed: Error: node.js checker: Neither "start" in the "scripts" section of "package.json" nor the "server.js" file were found.
I added below in package.json
"engines": {
"node": "^13.8.0",
"npm":"^6.13.4"
},
app.yaml file
# [START runtime]
runtime: nodejs
env: flex
service: dev
handlers:
- url: /.*
secure: always
script: auto
redirect_http_response_code: 301
automatic_scaling:
min_num_instances: 1
max_num_instances: 2
cool_down_period_sec: 60
cpu_utilization:
target_utilization: 0.80
package.json
"scripts": {
"start": "serve -s ./build",
"prestart": "npm i -g serve",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
}
cloudbuild.yaml
steps:
- name: gcr.io/cloud-builders/npm
entrypoint: 'npm'
args: [ install ]
- name: gcr.io/cloud-builders/npm
entrypoint: 'npm'
args: [ run, build, --prod ]
- name: gcr.io/cloud-builders/gcloud
entrypoint: 'npm'
args: [ app, deploy, '[public/app.yaml]', --version=$SHORT_SHA ]

Answer 1

If you're using the GAE standard, keep in mind that it currently supports Node.js 10 and 12 runtimes.
Also, as the official GCP documentation mentions:
By default, the runtime starts your application by running node
server.js. If you specify a start script in your package.json file,
the runtime runs the specified start script instead.
"scripts": {
"start": "node app.js"
}
Please, have a look at this GitHub repository, which contains a simple app for GAE standard, which you could use as a reference.
EDIT
You need to make sure that your start script is starting a web server that responds to HTTP requests on the port specified by the PORT environment variable, typically 8080 (link).
Here you can see an example for the GAE flexible environment, where the app.js contains
// Start the server
const PORT = process.env.PORT || 8080;
app.listen(PORT, () => {
console.log(`App listening on port ${PORT}`);
console.log('Press Ctrl+C to quit.');
});
While the package.json has
"scripts": {
"start": "node app.js"
}
This error can be reproduced when deploying that sample on GAE with Cloud Build after removing the start script or if the app is not starting the server.