SyncBackPro has built-in support for SFTP servers. No extra software is required.

SyncBackSE includes support for 
FTPS servers, but not SFTP servers.

SyncBackFree includes support for FTP servers but not FTPS or SFTP servers.

However, you can access SFTP using SyncBackSE/Free using 3rd party products such as 
WebDrive (commercial), and this article explains how to use Tunnelier. There is also a KB article on using WebDrive with SyncBack.

SyncBack Touch
An alternative to using SFTP is to use SyncBack Touch. SyncBack Touch is a cross-platform file server. It lets SyncBackPro and SyncBackSE copy files to and from another device, e.g. Windows, macOS, Linux or Android device, over the network (LAN or Internet) just like an FTP server. SyncBackFree cannot use SyncBack Touch.

install SyncBack Touch
 on the device you want to the access the files on and then create a profile in SyncBackPro/SE to use that device.

The advantages to using SyncBack Touch instead of FTP are that you can copy open/locked files, it's far simpler to set-up, doesn't have all the compatibility problems of FTP and doesn't require complex firewall/router configuration. SyncBack Touch also encrypts all communication by default.