SFTP plug in issue - Error copying the Appian document to the remote server

Hi 

I am trying to connect to SFTP server using the Send File over SFTP smart service using the public/private key set-up.

I have configured the below parameters of the smart service: 

External System Key: set to the value of external system setting

Use Per User Credentials: set to False

Private Key: set to .ppk document 

known hosts, server port, remote file path and source document are set to the required values or document respectively.

However, I am getting the below mentioned error: 

Could anyone please help me out here.

Thanks in advance.

Sonal