@Vardan_Harutyunyan Welcome to the Rockstor community forum.
Hopefully the following section in our official docs should help:
http://rockstor.com/docs/sftp/sftp.html
It give details of constraints that currently exist with regard to how Rockstor currently servers up sftp.
ie: in the section "The Rockstor SFTP System"
we have:
"The SFTP user must be a Rockstor user"
"The SFTP user must also be the owner of an exported SFTP share"
Hope that helps.