Hi @subash sharma ,
Atlas actually uses HBase as graph storage backend. From the stack trace it looks like, Atlas is not able to communicate with HBase. Please check if HBase(region servers) is down. Start HBase and restart Atlas.
you can even install HDP-2.6.5 which has atlas-0.8 version . : https://docs.hortonworks.com/HDPDocuments/HDP2/HDP-2.6.5/index.html
Hope this helps.