When setting a up a new server via Plesk on a Debian System, one of the first tasks is to enable SSH/SFTP users.

By default & security SSH permissions are off.

ssh user@host.name
Permission denied (publickey).

To change that, log into the server as su - via the Plesk SSH Interface - and update the settings in /etc/ssh/sshd_config

Also a good idea to make a backup, before changing anyting.

cd /etc/ssh/
cp sshd_config sshd_config.back

Change PasswordAuthentication and KbdInteractiveAuthentication to yes

nano sshd_config
...
# To disable tunneled clear text passwords, change to no here!
PasswordAuthentication yes
#PermitEmptyPasswords no

# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
KbdInteractiveAuthentication yes
...

The SSH daemon/service needs to be restarted:

sudo systemctl restart ssh

SSH/SFTP log-in should now work!