During the preflight checks, you may find the following error is displayed when checking server connectivity:
SFTP channel could not be opened
This is most likely because SFTP hasn't been enabled on your server. You can check that the following line is uncommented and available in /etc/ssh/sshd_config:
Subsystem sftp /usr/lib/openssh/sftp-server
Please note that any changes to your config file will only take effect once you restart your SSH service.