Some server providers, such as Amazon EC2 and Google Compute Engine, disable SSH password authentication by default. That is, you can only log in over SSH using public key authentication.
How to Enable SSH Password Authentication
SSH
To enable SSH password authentication, you must SSH in as root to edit this file. Then, change the line
sudo nano /etc/ssh/sshd_config
Then, change the line
PasswordAuthentication no
to
PasswordAuthentication yes
After making that change, restart the SSH service by running the following command as root:
sudo service ssh restart
Enable Logging In as root
Some providers also disable the ability to SSH in directly as root. In those cases, they created a different user for you that has sudo privileges (often named ubuntu). With that user, you can get a root shell by running the command:
sudo -i
If you instead want to be able to directly SSH in as root, again edit this file:
/etc/ssh/sshd_config
And change the line
PermitRootLogin no
to
PermitRootLogin yes
After making that change, restart the SSH service by running the following command as root:
sudo service ssh restart
If you enable this setting, don’t forget to set a strong password for root by running the command.
sudo passwd root