During the preflight checks, you may find one of the following errors are displayed when checking server connectivity:
SFTP channel could not be opened
could not start SFTP subsystem
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.