Here's the story
https://pjmedia.com/news-and-politics/rick-moran/2020/08/13/questions-raised-about-kamala-harriss-eligibility-to-become-vice-president-n787368
SO: because neither of her parents was a citizen at the time of her birth
AND because at the time of her birth being born in the U.S. was not by itself sufficient to qualify the newborn for citizenship
ARGUABLY
legally
she also was not a citizen at birth.
If not a natural-born citizen
then
she is not eligible to run for either President or Vice-President.
As that article makes clear
no U.S. court will ever bother hearing the case
but legally there IS a case.