Enabling SFTP is very easy and makes FTP protocol a lot more secure than traditional FTP because passwords are not sent in plain text which is susceptible to a lot of things such as packet sniffers, man-in-the-middle, etc.
Assuming VSFTPD has been installed (in the default location) edit the file /etc/vsftpd.conf and check there are security certificates installed and configured.
There are two entries which start with either:
Each specifying a file path to where the key and certificates are located.
The last step is to enable SFTP by adding the following line:
Save and exit the file and restart the FTP server
sudo /etc/init.d/vsftpd restart