DRS uses username and password to authenticate, not RSA keys. Ensure your SFTP server isn’t expecting the latter.
Also, the only SFTP server supported by Cisco with DRS these days is Prime Collaboration Deployment; they killed off generic OpenSSH support. This is important because Cisco can hide behind it as an excuse if you ever need to do a restore and the backup isn’t viable (DRS does not checksum/validate that a backup is intact after uploading it.)