Hi Saeed,
I would be so thankful if you guide me on how should I resolve this issue or where in the qlustar config files should I look for the source of the problem.
take a look at /etc/ssh/sshd_config: does it contain a line starting with AllowUsers? If so, only users listed there can connect.
On our cluster, only root is allowed on the head node, and I wouldn't want any other users there. Instead, we have a few login nodes for that purpose. But maybe on smaller clusters where setting aside a box just for logins is more expensive (relattively speaking), allowing users on the head node might be more attractive.
A.