Q:

Kubernetes not claiming persistent volume - "failed due to PersistentVolumeClaim is not bound: "task-pv-claim", which is unexpected."

I'm not sure why the persistent volume is not being claimed, or what steps I could take to further diagnose this? 
Should the claim size match the volume size? Should the volume size match the GCP volume size?
This is so difficult to test and figure out... 
My goal here is just to be able to create a Wordpress instance with even a single replica as long as it would support rolling deployments....
Output of kubectl get pods:
NAME                         READY     STATUS    RESTARTS   AGE
wordpress-1546832918-mz4rt   0/3       Pending   0          47m
wordpress-1546832918-p0s1s   0/3       Pending   0          47m

Output of kubectl describe pods:
...truncated...
Events:
  FirstSeen LastSeen    Count   From            SubObjectPath   Type        Reason          Message
  --------- --------    -----   ----            -------------   --------    ------          -------
  47m       3s      168 default-scheduler       Warning     FailedScheduling    [SchedulerPredicates failed due to PersistentVolumeClaim is not bound: "task-pv-claim", which is unexpected., SchedulerPredicates failed due to PersistentVolumeClaim is not bound: "task-pv-claim", which is unexpected.]

Output of kubectl get pvc:
NAME            STATUS    VOLUME    CAPACITY   ACCESSMODES   STORAGECLASS   AGE
task-pv-claim   Pending                                      manual         4h

Output of kubectl get pv:
NAME      CAPACITY   ACCESSMODES   RECLAIMPOLICY   STATUS      CLAIM     STORAGECLASS   REASON    AGE
pv0001    10Gi       RWX           Retain          Available             manual                   4h

production.yaml
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: wordpress
  labels:
    app: wordpress
spec:
  replicas: 2
  selector:
    matchLabels:
      app: wordpress
  template:
    metadata:
      labels:
        app: wordpress
    spec:
      terminationGracePeriodSeconds: 30
      containers:
        - image: eu.gcr.io/abcxyz/wordpress:deploy-1502807720
          name: wordpress
          imagePullPolicy: "Always"
          env:
            - name: WORDPRESS_HOST
              value: localhost
            - name: WORDPRESS_DB_USERNAME
              valueFrom:
                secretKeyRef:
                  name: cloudsql-db-credentials
                  key: username
          volumeMounts:
            - name: wordpress-persistent-storage
              mountPath: /var/www/html
        - image: eu.gcr.io/abcxyz/nginx:deploy-1502807720
          name: nginx
          imagePullPolicy: "Always"
          ports:
            - containerPort: 80
              name: nginx
          volumeMounts:
            - name: wordpress-persistent-storage
              mountPath: /var/www/html
              readOnly: true
        - image: gcr.io/cloudsql-docker/gce-proxy:1.09
          name: cloudsql-proxy
          command: ["/cloud_sql_proxy", "--dir=/cloudsql",
                    "-instances=abcxyz:europe-west1:wordpressdb2=tcp:3306",
                    "-credential_file=/secrets/cloudsql/credentials.json"]
          volumeMounts:
            - name: cloudsql-instance-credentials
              mountPath: /secrets/cloudsql
              readOnly: true
            - name: ssl-certs
              mountPath: /etc/ssl/certs
            - name: cloudsql
              mountPath: /cloudsql
      volumes:
        - name: wordpress-persistent-storage
          persistentVolumeClaim:
            claimName: "task-pv-claim"
        - name: cloudsql-instance-credentials
          secret:
            secretName: cloudsql-instance-credentials
        - name: ssl-certs
          hostPath:
            path: /etc/ssl/certs
        - name: cloudsql
          emptyDir:

pVolume.yaml
apiVersion: "v1"
kind: "PersistentVolume"
metadata:
  name: "pv0001" 
spec:
  storageClassName: manual
  capacity:
    storage: "10Gi" 
  accessModes:
    - "ReadWriteMany"
  gcePersistentDisk: 
    fsType: "ext4" 
    pdName: "wordpress-disk" 

pVolumeClaim.yaml
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: task-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi

A:

The spec.accessModes of your persistent volume claim has to match that in the persistent volume. Try change both of them to the same value.
If that didn't work, you can add the spec.selector definition to your persistent volume claim definition, by updating it to match your persistent volume metadata.labels like this:
apiVersion: "v1"
kind: "PersistentVolume"
metadata:
  name: "pv0001" 
  labels:
    name: "pv0001" # can be anything as long as it matches the selector in the pvc
spec:
  ...

----
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: task-pv-claim
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi
  selector:
    matchLabels:
      name: "pv0001" 

The spec.selector serves as a filter to ensure that only PV with the specified labels are matched.

