I know this is a simple process, but I was in the process of building a testing domain, so figured I would go ahead and screenshot the process, and add some simple notes to it. I hope someone finds it useful.
Active Directory Domain Services Installation Wizard (Dcpromo.exe) issues
Explains the error above, plus any other issues you may encounter.
Once the server reboots, you will need to logon to the server as a Domain Administrator. Which used to be your local admin account on the server. Just make sure to logon to the domain.
You can now begin configuring the domain.