Kamala Harris can beat Trump if Senator Harris steps forward, not presidential candidate Harris. I always found her very impressive in Senate hearings. She could roast Trump in a debate. Trump would be a fool to debate her. But I repeat myself.
I think Trump would be more than happy to debate her. She's a woman, and a woman of color to boot. He sees her as inferior, and his ego wouldn't let him pass up the chance to belittle her. Which is what he'd do. He would treat her like he treated Hillary. Would he win that contest? Probably not. Would she win the election?
I have concerns. There's a reason she's VP and not POTUS. She was out of the primary before Iowa, and hasn't shown since taking office that the party really screwed up by not selecting her as the candidate. The only other candidate from the 2020 race that's in the administration is Buttigieg at Transportation Secretary, and from what I've seen, he would be the better choice than Harris. Whenever he appears on a cable news show he's even-keeled, well prepared, and when it's a hostile network like Fox he wipes the floor with everyone who tries a "gotcha" question.