With FTP Session Error - Renci.SshNet.Common.SshException: Invalid private key file

I have to use a Username, Password and Private Key (.ppk) to access an SFTP. While trying to use the “With FTP Session” activity, I get the following error:

Renci.SshNet.Common.SshException: Invalid private key file

Does this activity accept the use of Private Keys or is the Private Key in the wrong format?