The server provider YottaHost by default only allows SSH access to the VPS using public key authentication, SSH password access is disabled.
SFTP is a protocol that works over SSH, so this means that SFTP using passwords will not work by default when SSH password authentication is disabled.
To enable SSH password authentication, you must login as root via SSH (VNC) to edit this file:
/etc/ssh/sshd_config
Then, change the line
PasswordAuthentication no
to
PasswordAuthentication yes
For Ubuntu you also need to make the following changes
#PermitRootLogin prohibit-password
to
PermitRootLogin yes
After making that change, restart the SSH service by running the following command as root:
systemctl restart sshd
