Question

Why the efficiency of an engine cannot be 100%?

Answer 1

No machine can operate at 100 percent efficiency because some of the energy input will always be used to overcome the force of gravity and the effects of friction and air resistance.

Answer 2

No, because the engine can't convert enough heat. 100%? That's way too much. That is theoretically impossible. Its important to consider the equation, this will give you the better picture as to why it can't be. In fact, the maximum efficiency CANNOT be more than the Carnot limit. Its the 2nd Law of Thermodynamics. 
nth ≤ 1 - (Tc/Th) 

I hope this helped.