This error message indicates that the SFTP server requires the user to authenticate using both a password and a public key. To do this, call the
Login method overload that accepts password and private key (which is needed to perform public key authentication):
var sftp = new Sftp();
var key = new SshPrivateKey("mykey.ppk", "key_password");
sftp.Login("user01", "password", key);
However, this assumes you actually have the private key.