Setting up app.yaml file for Node.js and React.js deployment to App Engine

I'm trying to deploy my project which has a server-side directory and a client-side. I'm also using pm2 to boot up the services for both sides... Here is the folder structure directory:

MyProject
+-- server
|   --- app.js
|   --- app.yaml
|   --- package.json
+-- client
|   +-- build
|       --- index.html
|   --- package.json
|....
--- .gitignore

Server script of package.json

{...
"scripts": {
    "client": "npm start --prefix ../client",
    "deploy": "gcloud app deploy app.yaml",
    "dev": "concurrently \"npm run build\" \"npm run client\"",
    "start": "pm2 start app.config.json"
  },
...
}

app.config.json file because I'm using pm2:

{
    "apps" : [
        {
            "name"      : "clientApp",
            "script": "node",
            "args"    : "../client/scripts/start.js"
        },
        {
            "name"      : "serverApp",
            "script"    : "node",
            "args": "./bin/www"
        }
    ]
  }

app.js

app.use(express.static(path.join(__dirname, '../client/build')));

This is the setup for the Server. Now here is the Client side. package.json

"scripts": {
    "start": "pm2 start scripts/start.js --name clientApp",
    "build": "node scripts/build.js",
    "serve": "serve -s build -l 8080",
  },
  "proxy": "http://localhost:5000",

Most importantly, the app.yaml:

runtime: nodejs
env: flex

handlers:
  - url: /api/.*
    script: auto
  - url: /
    static_files: client/build/index.html
    upload: client/build/index.html
  - url: /
    static_dir: client/build
  - url: /static
    static_dir: client/build/static
  - url: /.*
    secure: always
    redirect_http_response_code: 301
    script: auto

manual_scaling:
  instances: 1

resources:
  cpu: 2
  memory_gb: 8.0
  disk_size_gb: 20

The error I'm getting is Application startup error! Code: APP_CONTAINER_CRASHED, why is this happening? Did i source the client side directory incorrectly in the app.yaml???

ERROR: (gcloud.app.deploy) Error Response: [9] 
Application startup error! Code: APP_CONTAINER_CRASHED
yarn run v1.17.3
$ pm2 start app.config.json
...
...
...
[PM2] Spawning PM2 daemon with pm2_home=/root/.pm2
[PM2] PM2 Successfully daemonized
[PM2][WARN] Applications clientApp, serverApp not running, starting...
[PM2] App [clientApp] launched (1 instances)
[PM2] App [serverApp] launched (1 instances)
┌─────┬──────────────┬─────────────┬─────────┬─────────┬──────────┬────────┬──────┬───────────┬──────────┬──────────┬──────────┬──────────┐
│ id  │ name         │ namespace   │ version │ mode    │ pid      │ uptime │ \u21ba    │ status    │ cpu      │ mem      │ user     │ watching │
├─────┼──────────────┼─────────────┼─────────┼─────────┼──────────┼────────┼──────┼───────────┼──────────┼──────────┼──────────┼──────────┤
│ 0   │ clientApp    │ default     │ N/A     │ fork    │ 50       │ 0s     │ 0    │ online    │ 0%       │ 28.2mb   │ root     │ disabled │
│ 1   │ serverApp    │ default     │ N/A     │ fork    │ 56       │ 0s     │ 0    │ online    │ 0%       │ 25.4mb   │ root     │ disabled │
└─────┴──────────────┴─────────────┴─────────┴─────────┴──────────┴────────┴──────┴───────────┴──────────┴──────────┴──────────┴──────────┘
Done in 2.05s.

error Command failed with exit code 1.