You run a command like this:
gcloud container clusters get-credentials standard-cluster-1 --region us-central1-a
But you receive one of the following messages:
"Fetching cluster endpoint and auth data.
ERROR: (gcloud.container.clusters.get-credentials) ResponseError: code=403, message=Request had insufficient authentication scopes."
ERROR: (gcloud.projects.describe) User [email@example.com] does not have permission to access projects instance  (or it may not exist): Request had insufficient
What should you do?
Here is a basic solution that is ideal for a proof-of-concept. It is not ideal for an enterprise environment. Ideally the server you are using is either secure, or you plan on terminating it in an hour or two.
1. Log into the GCP console
2. Go to Compute engine in the web UI.
3. Stop the VM.
4. Click on the hyperlinked name of the VM.
5. Click "Edit"
6. Check the option to "Allow full access to all Cloud APIs" if it was not checked. (If it was chosen, these directions will not help you. Please try to find something else.)
7. Click Save
8. Restart the VM.
9. Try again.