Hi there!
Correct me if I'm wrong, but I believe the parameters for your /etc/ssh/sshd_config would look something like the following:
PermitRootLogin prohibit-password
This will allow root to login with keys only, not password.
PubkeyAuthentication yes
This allows public key authentication.
AuthorizedKeysFile .ssh/authorized_keys
That tells you where the authorized keys are stored.
PasswordAuthentication no
This will disable password login.
Then restart your sshd service with this command:
systemctl restart sshd
Hope this helps!