Former President Barack Obama has not endorsed Kamala Harris’ presidential bid because he doesn’t believe she can defeat Donald Trump, according to a source close to the Biden family. After Joe Biden’s unexpected resignation from the race on Sunday and his swift endorsement of Vice President Harris, most Democratic leaders have rallied behind her. However, Obama remains a notable exception. “Obama’s very upset because he knows she can’t win,” the Biden family source told The Post. “Obama knows she’s just incompetent—the border czar who never visited the border, saying that all migrants should have health insurance. She cannot navigate the…