I am trying to experimentally run a Postgres Service on a local Kubernetes cluster consisting of 2 Ubuntu-18.04 machines.

My postgres pod is stuck in ContainerCreating, and kubectl describe pod postgres gave me this message:

Events:
  Type     Reason                  Age              From                Message
    ----     ------                  ----             ----                -------
    Normal   Scheduled               14s              default-scheduler   Successfully assigned default/postgres-57b4695bc9-8wklp to cumulusg2
    Warning  FailedCreatePodSandBox  11s              kubelet, cumulusg2  Failed to create pod sandbox: rpc error: code = Unknown desc = [failed to set up sandbox container "832ee34a687d8a1aabb92b57ec6b6b5b8d5f55889c996c2bd4bc4ddcb106fdd2" network for pod "postgres-57b4695bc9-8wklp": networkPlugin cni failed to set up pod "postgres-57b4695bc9-8wklp_default" network: error getting ClusterInformation: Get "https://10.96.0.1:443/apis/crd.projectcalico.org/v1/clusterinformations/default": x509: certificate signed by unknown authority (possibly because of "crypto/rsa: verification error" while trying to verify candidate authority certificate "kubernetes"), failed to clean up sandbox container "832ee34a687d8a1aabb92b57ec6b6b5b8d5f55889c996c2bd4bc4ddcb106fdd2" network for pod "postgres-57b4695bc9-8wklp": networkPlugin cni failed to teardown pod "postgres-57b4695bc9-8wklp_default" network: error getting ClusterInformation: Get "https://10.96.0.1:443/apis/crd.projectcalico.org/v1/clusterinformations/default": x509: certificate signed by unknown authority (possibly because of "crypto/rsa: verification error" while trying to verify candidate authority certificate "kubernetes")]
    Normal   SandboxChanged          8s (x2 over 9s)  kubelet, cumulusg2  Pod sandbox changed, it will be killed and re-created.

The error message confuses me and I am not sure where to start, so I'll lay out my process up to this point. To initialize the cluster, I used

sudo kubeadm init --pod-network-cidr=192.168.0.0/16
mkdir -p $HOME/.kube
sudo cp -i /etc/kubernetes/admin.conf $HOME/.kube/config
sudo chown $(id -u):$(id -g) $HOME/.kube/config

,then the kubeadm join command, and after that:

kubectl apply -n kube-system -f "https://cloud.weave.works/k8s/net?k8s-version=$(kubectl version | base64 |tr -d '\n')"

To create the Postgres Database, I used 3 yaml files:

postgres-configmap.yaml

apiVersion: v1
kind: ConfigMap
metadata:
  name: postgres-config
  labels:
    app: postgres
data:
  POSTGRES_DB: postgresdb
  POSTGRES_USER: postgresadmin
  POSTGRES_PASSWORD: admin123

postgres-volumes.yaml

kind: PersistentVolume
apiVersion: v1
metadata:
  name: postgres-pv-volume
  labels:
    type: local
    app: postgres
spec:
  storageClassName: manual
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteMany
  hostPath:
    path: "/mnt/data"
---
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: postgres-pv-claim
  labels:
    app: postgres
spec:
  storageClassName: manual
  accessModes:
    - ReadWriteMany
  resources:
    requests:
      storage: 1Gi

and postgres-deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: postgres
spec:
  replicas: 1
  selector:
    matchLabels:
      app: postgres
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres
          image: postgres:latest
          imagePullPolicy: "IfNotPresent"
          ports:
            - containerPort: 5432
          envFrom:
            - configMapRef:
                name: postgres-config
          volumeMounts:
            - mountPath: /var/lib/postgresql/data
              name: postgredb
      volumes:
        - name: postgredb
          persistentVolumeClaim:
            claimName: postgres-pv-claim
---
apiVersion: v1
kind: Service
metadata:
  name: postgres
  labels:
    app: postgres
spec:
  type: NodePort
  ports:
   - port: 5432
  selector:
   app: postgres