The message here is clear:
org.elasticsearch.bootstrap.StartupException: java.lang.RuntimeException: can not run elasticsearch as root
This is because you are executing the startup script as root. Let us not do that to avoid this problem.
Next, would you please move your Elasticsearch installation off of /
and put it in /home/elastic/
? If this still causes trouble, would you execute:
$ bash -x /home/elastic/elasticsearch-6.2.1/bin/elasticsearch
and we will start debugging from there.