You don't actually create the title in order to reform your religion. There is a special button in the Religion tab that you should use instead, which will reform the religion and automatically create the Religious head title for you.
You can find the button at the top left hand corner of the interface. It should say "Reform the Germanic Faith" when you hoover over the icon.
You can find the button at the top left hand corner of the interface. It should say "Reform the Germanic Faith" when you hoover over the icon.