Appeal to motive fallacy. Even if it's true that he's motived by being too "Too Chickenshit to debate." It doesn't argue that he should. Not to mention that fact that Trump has historically dominated in debates that it was said he'd get stomped in.
Trump doesn't feel bound by the Queensbury rules in debate like the other establishment clowns in the GOP do. He just goes up there and says what he wants, irrespective of how offensive it is. That's what people like about him. He's real and not their to kiss ass and impress establishment and leftwing GOP idiots.