In ubuntu elasticsearch kibana.yml
file not found at location /opt/kibana/config/kibana.yml
which is usually the default path for kibana.yml
file.
Older 4.x versions used the path /opt/kibana/config/kibana.yml
but that has been changed in 5.x to /etc/kibana/kibana.yml
(which is consistent with Elasticsearch also).
So look for /etc/kibana/kibana.yml
.