Elasticsearch listening on /127.0.0.1:9001 . Also shows Main PID: 12471 (code=exited, status=1/FAILURE) error messages- ubuntu server 16.04.3

Yes. But it happens that people are sleeping or doing other stuff.
Read this and specifically the "Also be patient" part.

Again, please format your code.

I can see by chance that Sonar is supposed to use a 5.6 version but you are trying to install a 6.1 version. There is a chance that this is not going to work.

Anyway, to install elasticsearch, please follow the guide @warkolm linked to previously.