1.16.15 looks like your Kubernetes version. Assuming you installed ECK using the Helm chart and you are running ECK 1.4.0 (you can check this by running helm list --all-namespaces | grep eck-operator) you could be running into a known issue with the validating webhook.
If the above assumptions are correct (ECK 1.4.0 installed using Helm), you can try patching the validating webhook as follows before retrying the volume expansion operation.
WEBHOOK=$(kubectl get validatingwebhookconfiguration --no-headers -o custom-columns=NAME:.metadata.name | grep 'k8s.elastic.co')
kubectl patch validatingwebhookconfiguration "$WEBHOOK" --patch='{"webhooks": [{"name": "elastic-es-validation-v1.k8s.elastic.co", "matchPolicy": "Exact"}, {"name": "elastic-es-validation-v1beta1.k8s.elastic.co", "matchPolicy": "Exact"}]}'