Kubernetes AutoScaler Not Scaling, HPA shows target <unknown>

As Heapster is deprecated in later version(v 1.13) of kubernetes, You can expose your metrics using metrics-server also, Please check following answer for step by step instruction to setup HPA:

How to Enable KubeAPI server for HPA Autoscaling Metrics

To answer the question directly, if you have set up an HPA resource and using the metrics server, and ran into this error.

This error means that in your pods, which may have more than one container, either one or both of the containers have not defined resource requests:

Resources:
     requests:
       cpu: <this is missing! Add it>

Check that all containers have resource requests defined.

To add edit and add resources directly, use kubectl edit <resource>

Kubernetes may not allow you to update if your using helm even with the force flag.

A note, this may cause downtime if you have not set PodDisruptionBudgets so set those before you ran your edit