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Fedora 28 followed Ubuntu's lead and did away with the root password (see item #2), which among other beneficial effects means it's not possible to log in to the system as root. Root logins are a huge security exposure and should never be done for any reason. Executing permissions-elevated operations from a normal user account via sudo is the preferred method of accessing root capabilities.

However, the problems running dm-tool would seem to indicate that the lightdm package is not installed, as it's the source of that utility. If you indeed are using or attempting to use lightdm instead of the Fedora default gdm as your display manager, then sudo dnf install lightdm should make the dm-tool utility available.