Here the words “of Xerxes” imply that Darius’ father was a man named Xerxes. That would identify Darius as King Darius I of Persia. However, this is impossible. Darius I was the father of Xerxes (not his son), and he lived long after DaniEl, so DaniEl couldn’t have written about him.
So the two words “of Xerxes” may be spurious.
Perhaps they started as a marginal note, added by someone who thought they were somehow helping readers to identify Darius I. Then the note may then have been transposed into the text upon copying (this is how several spurious additions ended up in the Bible, which modern translators usually remove). Alternatively, ‘Xerxes’ may have been a common family or a title name given to Darius the Mede’s actual father, whoever he was.