Elasticsearch installation password issue?

HI, I am installing elasticsearch 5.6.3 on ubuntu.
I am using command service elasticsearch start
user is ubuntu and the terminal ask for my password
I have tried many default and I can't surpass it.
Please advise.

Elasticsearch 5 is End Of Life so not a good version to start installing now. https://www.elastic.co/support/eol

Not knowing the password your computer is asking for is nothing to do with Elasticsearch.