Trump has no debating skills whatsoever, and he knows it. Despite his bluster, he's an intimidated child in any debate because he can't even remember the lies he told just last week. Debates tend to be among the most heavily fact checked of any public speaking events by pollitical candidates, so he knows his answers would be eviscerated by any credible media within half-an-hour after the debate.
He thought he could physically intimidate Hillary Clinton by sneaking up behind her and looming over her in their debate. It didn't work. His first debate with Biden he wouldn't shut up, kept interrupting and yelling over him, and made the whole thing a circus. The press slammed him for his childishness, and he backed out of his second debate with Biden, because he knew he wouldn't be allowed to get away with such childish rudeness again. When he feels even the least bit challenged, he loses every bit of his already limited self control.
As a general rule, only people who actually know facts are good at debating, and Trump doesn't know shit. He'd be completely lost on stage, unless he was guaranteed that the answer to every question would be Person, Woman, Man, Camera, TV.