Hello @bhasat3022, ES version 6.8 should be supported under version 6.0+. Just to confirm, have you picked version 6.0+ from the data source settings dropdown?
Also, have you checked following threads with couple of problems-solutions:
https://community.grafana.com/t/elasticsearch-time-field-name/2184/5
https://community.grafana.com/t/how-can-i-use-elasticsearch-data-source-if-i-do-not-have-time-field-name/4510