It means that heapster is not properly configured.

You need to make sure that heapster is running on kube-system namespace, and check if the /healthz endpoint is ok:

$ export HEAPSTER_POD=$(kubectl get po -l k8s-app=heapster -n kube-system -o jsonpath='{.items[*].metadata.name}')
$ export HEAPSTER_SERVICE=$(kubectl get service/heapster --namespace=kube-system -o jsonpath="{.spec.clusterIP}")
$ curl -L "http://${HEAPSTER_SERVICE}/healthz"
ok

Then, you can check if the metrics API is available:

$ curl -L "http://${HEAPSTER_SERVICE}/api/v1/model/metrics/"
[
  "cpu/usage_rate",
  "memory/usage",
  "cpu/request",
  "cpu/limit",
  "memory/request",
  "memory/limit"
 ]

If it's not returning as above, take a look at container logs for errors:

$ kubectl logs -n kube-system ${HEAPSTER_POD} --all-containers

Although, keep in mind that Heapster is a deprecated project and you may have problems when running it in recent Kubernetes versions.

See Heapster Deprecation Timeline:

| Kubernetes Release  | Action              | Policy/Support                                                                   |
|---------------------|---------------------|----------------------------------------------------------------------------------|
| Kubernetes 1.11     | Initial Deprecation | No new features or sinks are added.  Bugfixes may be made.                       |
| Kubernetes 1.12     | Setup Removal       | The optional to install Heapster via the Kubernetes setup script is removed.     |
| Kubernetes 1.13     | Removal             | No new bugfixes will be made.  Move to kubernetes-retired organization.          |

Since Kubernetes v1.10, the kubectl top relies on metrics-server by default.

CHANGELOG-1.10.md:

• Support metrics API in kubectl top commands. (#56206, @brancz)

This PR implements support for the kubectl top commands to use the metrics-server as an aggregated API, instead of requesting the metrics from heapster directly. If the metrics.k8s.io API is not served by the apiserver, then this still falls back to the previous behavior.

It's better to use a kubectl version v1.10 or above, as it fetches the metrics from metrics-server.

However, beware of kubectl Version Skew Policy:

kubectl is supported within one minor version (older or newer) of kube-apiserver

Check your kube-apiserver version before choosing your kubectl version.