The error message you typed as the subject of this thread is what you see when you type sudo as root.

See, to become root on the N900, typically you install the root package and type "sudo gainroot" from the terminal. Now you're root and you can do anything you want.

What you can't do is type sudo again since you're already root it makes no sense to type this again.

The actual error message is telling you that the root user is not in the sudoers file so the command failed. This is because the sudoers file lists a bunch of user names that are allowed to type sudo to become root. Root is not in this file because root will never need to type sudo to be come root. Root is already root.

Got it?