If you put Fractal Harness onto the battlefield without casting it, or if you choose 0 as the value of X, the token you create will be 0/0. Unless something else is raising its toughness, that token will enter the battlefield and die before you could take any actions.

To double the number of +1/+1 counters on a permanent, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.