Getting error while trying to login SFTP with username and privatekey?

Hi,

I am using rebex dll versision 2.0.5584.0 for SFTP conncetion. I am successfully login with username and password. But when I try to login with username and privateKey I am getting the below exception.

No suitable authentication method is supported. Supported methods: 'password,keyboard-interactive'.

Please below code:

SshPrivateKey privateKey = null;
privateKey = new SshPrivateKey(ppkFilename, password);
var sftp = new Sftp();
sftp.Connect(hostName);
ftp.Login(username, privateKey); //Here I am getting error.

I am using Rebex Tiny SFTP Server v.1.03.
Please help.