Yes.
This sounds like a dumb, arbitrary rule, we know. But bringing a half-finished manuscript into NaNoWriMo all but guarantees a miserable month. You'll simply care about the characters and story too much to write with the gleeful, anything-goes approach that makes NaNoWriMo such a creative rush. Give yourself the gift of a clean slate, and you'll tap into realms of imagination and intuition that are out-of-reach when working on pre-existing manuscripts.