Without breaking some additional Laws of physics, the answer is no.
The truck has inertia meaning that the truck will follow Newton's 1st law until an adequate force moves it as per his 2nd law. If your fist does not have enough force to move the truck, but cannot be stopped by an equal and opposite reaction, then you face the age old question of what happens when an unstoppable force meets an immovable wall.
The only logical outcome here (despite sounding illogical), is that your hand passes harmlessly through the truck. Hurting neither you nor the vehicle. ... well, you might still hurt the vehicle as much as punching one normally does since you still have the force of your punch you can apply to it as per the 2nd law, but this is very little by vehicular movement standards.
Now if you can also violate Newton's 1st Law of motion, then your punch can ignore the truck's inneria allowing you to send it flying at 6-21 m/s depending on how good your jab is. Depending on how you interpret this one, a second possible outcome is that you can only send the few square inches that contacted your fist at that speed. Because the contact area is so small in this case, you'd leave a deep dent in the vehicle like it got hit by a fire hydrant at highway speeds, and you'd probably flip the thing pretty nicely, but if you want any range, you're better off picking it up and throwing it like a giant discus.
Now let's say you can also violate Newton's 2nd Law of motion. You can now accelerate things faster than you hit them completely destroying any concept of conservation of energy. So if you choose, your 6 m/s grandma punch could accelerate the target at the speed of light. For reference each kg of matter contains the equivalent energy of a 21 megaton hydrogen bomb when accelerated to that speed.
Again there are a few ways to interpret this which are significantly different in scale: If we again assume that this affect only happens at the point of impact, then things are not so bad. As soon as your fist touches the truck, molecules become so excited that they are turned into energy with 142 times the efficiency of a hydrogen fusion reaction. Your first assumption would be that you'd level the city, but thankfully your fist is going much much slower than the explosion you are creating meaning that you'd only accelerate the truck as a whole until it is moving faster than your hand at which point there is no more contact to add to the reaction. This would send the truck hurreling off just slightly faster than your punch leaving the truck a smoldering mess, but the city still standing.
Now for the real nasty question... what if you sent the whole truck flying off at the speed of light? I hope we are all picturing relativistic baseball now. The maximum legal weight a fully loaded 18 wheeler truck is allowed to be in the US is 80,000 lbs (36,287 kg) If you can convert the whole truck into energy you're looking at a 762 Gigaton explosion or roughly 500 times the force of the yellowstone supervolcano eruption that nearly killed off mankind 640,000 years ago. Thats a few orders of magnitude shy of the Chicxulub impact, but still not very good for anything that enjoys being alive.
Since this part is technically getting off topic, I'm gonna skip the whole contact only thing and pretend your "Hero?" decides to punch the Earth into light speed. The Mass of the Earth is 5.972 × 10^24 kg meaning that punching it would create a force equal to a 1.254 × 10^26 megaton explosion which is only two orders of magnitude shy of a type 1a supernova.