While you are highly encouraged to use SSH keys for authentication when connecting to a server, you can enable access via password for the root account.
You are seeing that error as that line is incorrect. That is a command to run, not a line to put in the file.
First, to permit root logins with a password, you would need to add the following to your
Then to actually set the password for the root account, you will need to run this command:
It will prompt you to enter a new password for the root user.
SSH keys provide a more secure way of logging into a virtual private server with SSH than using a password alone. With SSH keys, users can log into a server without a password. This tutorial explains how to generate, use, and upload an SSH Key Pair.