Enable root login with password on the server. To do so, you need to edit the file /etc/ssh/sshd_config.
Modify the lines starting with "PermitRootLogin" like this: #PermitRootLogin prohibit-password
PermitRootLogin yes
Restart the sshd server: sudo systemctl restart sshd
On the client, copy the public key files to the server with the ssh-copy-id command: ssh-copy-id root@server
On the server, disable root login with password and enable root login with public key authentication:
