I have been following this guide - Connecting from Kubernetes Engine

And having an errors after all:

$ kubectl describe pod | grep Warning
  Warning  FailedMount            4m (x15 over 19m)  kubelet, gke-bar-dev-default-pool-a6045c50-dg5z  MountVolume.SetUp failed for volume "cloudsql-instance-credentials" : secrets "cloudsql-instance-credentials" not found
  Warning  FailedMount            3m (x7 over 17m)   kubelet, gke-bar-dev-default-pool-a6045c50-dg5z  Unable to mount volumes for pod "bar-dev-556d7c4f6f-5c2nx_default(9a06b84a-7dcb-11e8-bca8-42010a8e0060)": timeout expired waiting for volumes to attach/mount for pod "default"/"bar-dev-556d7c4f6f-5c2nx". list of unattached/unmounted volumes=[cloudsql-instance-credentials]
  Warning  Failed                 2m                 kubelet, gke-bar-dev-default-pool-a6045c50-dg5z  Error: secrets "cloudsql-db-credentials" not found

My deployment.yaml file:

apiVersion: apps/v1beta2
kind: Deployment
metadata:
  name: bar-dev
  labels:
    app: bar-dev
spec:
  replicas: 1
  selector:
    matchLabels:
      app: bar-dev
  template:
    metadata:
      labels:
        app: bar-dev
    spec:
      containers:
        - name: nginx
          image: gcr.io/foo/bar-dev-nginx:v9
          ports:
          - containerPort: 80
          readinessProbe:
            httpGet:
              path: /health-check
              port: 80
            initialDelaySeconds: 10
            periodSeconds: 60
          env:
            - name: POSTGRES_DB_HOST
              value: 127.0.0.1:5432
            # [START cloudsql_secrets]
            - name: POSTGRES_DB_USER
              valueFrom:
                secretKeyRef:
                  name: cloudsql-db-credentials
                  key: username
            - name: POSTGRES_DB_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: cloudsql-db-credentials
                  key: password
            # [END cloudsql_secrets]
        - name: php-fpm
          image: gcr.io/foo/bar-dev-php-fpm:v9
          ports:
            - containerPort: 9000
          env:
            - name: POSTGRES_DB_HOST
              value: 127.0.0.1:5432
            # [START cloudsql_secrets]
            - name: POSTGRES_DB_USER
              valueFrom:
                secretKeyRef:
                  name: cloudsql-db-credentials
                  key: username
            - name: POSTGRES_DB_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: cloudsql-db-credentials
                  key: password
            # [END cloudsql_secrets]
        # [START proxy_container]
        - name: cloudsql-proxy
          image: gcr.io/cloudsql-docker/gce-proxy:1.11
          command: ["/cloud_sql_proxy",
                    "-instances=foo:us-east1:bar-dev=tcp:5432",
                    "-credential_file=/secrets/cloudsql/credentials.json"]
          volumeMounts:
            - name: cloudsql-instance-credentials
              mountPath: /secrets/cloudsql
              readOnly: true
        # [END proxy_container]
      volumes:
        - name: cloudsql-instance-credentials
          secret:
            secretName: cloudsql-instance-credentials

What is causing this issue and how to resolve it?