Let's say you are attempting to use the oc exec command to run a command in a container in a pod.

oc exec pod/my-pod-78f97cfd6d-dptdq -c my-container -- df

And the following is being returned.

error: unable to upgrade connection: container not found ("my-container")

I typically find that there is some issue causing the pod to be restart over and over and over again. In this scenario, you of course would want to determine why the pod keeps restarted over and over again. Check out my article on resolving CrashLoopBackOff.

~]$ oc get pods
NAME                       READY   STATUS              RESTARTS           AGE
my-pod-78f97cfd6d-dptdq    0/1     CrashLoopBackOff    8260 (5m23s ago)   19d