The log message means that Fluentd cannot reach your ElasticSearch cluster. Based on the link that you shared.
You need to apply this service to expose your ElasticSearch cluster on port 9200 as elasticsearch-logging on the same namespace where your Fluentd pod is running. So it appears that elasticsearch-logging is missing. You can find out more details with:
kubectl -n kube-system get svc
and
kubectl -n kube-system describe svc elasticsearch-logging
Another issue could be that DNS is not resolvable within the namespace which could be coredns or kube-dns. You could try shelling into a pod in the same namespace:
kubectl -n kube-system exec -it <pod-in-kube-system> sh
Then inside
curl elasticsearch-logging:9200
