$ export KOPS_STATE_STORE=s3://BUCKET_NAME

$ kops validate cluster
Using cluster from kubectl context: CONTEXT_NAME

Validating cluster CLUSTER_NAME

Error: validation failed: unexpected error during validation: error listing nodes: Unauthorized


[SOLUTION_1]
$ kops update cluster --name CLUSTER_NAME --yes --admin
I0512 04:34:50.707754   40394 dns.go:235] Pre-creating DNS records
I0512 04:34:50.708532   40394 update_cluster.go:322] Exporting kubeconfig for cluster
kOps has set your kubectl context to kops.aws.k8s.com
Cluster changes have been applied to the cloud.
Changes may require instances to restart: kops rolling-update cluster

$ kops validate cluster --state=s3://BUCKET_NAME
Using cluster from kubectl context: CONTEXT_NAME

Validating cluster CLUSTER_NAME

INSTANCE GROUPS
NAME                            ROLE            MACHINETYPE     MIN     MAX     SUBNETS
xxxx                            xxxx            xxxxxxxxxxx     xxx     xxx     xxxxxxx
xxxx                            xxxx            xxxxxxxxxxx     xxx     xxx     xxxxxxx

NODE STATUS
NAME                    ROLE            READY
i-Instance_ID           control-plane   True
i-Instance_ID           node            True

Your cluster CLUSTER_NAME is ready
