The Subjects of Ymir (ユミルの民 Yumiru no Tami?) are an ethnic group descended from Ymir Fritz and the only race that was capable of becoming Titans
Since Eren is capable of turning into a Titan, he has to be a descendant of Ymir Fritz.
Only those with royal blood—the Fritz or Reiss royal families—were able to use the true power of the Founding Titan
So, how is that Eren isn't of royal bloodline, if he's a descendant of Ymir Fritz, who is the first ancestor of the Ymir royal family?