If Venus had moon before and it fell back to Venus, why don't I see a unique, very big crater? Does lack of a very big crater support the idea that Venus never had big moons before? 

If not, why can't I find a very big crater from the moon impact?
Venus has volcanism and weather, and those can cover or erode craters.  There are relatively few impact craters on Venus today, indicating that the surface of Venus is at most 600 million years old -- meaning that any older surface features have been erased.  The Earth's surface is even younger.

During the Late Heavy Bombardment, in the very early history of the Solar System, there was a period of time when many asteroids hit the Moon, creating many large impact craters.  It seems likely that the Earth and Venus got hit by many asteroids then, too.  Yet, no evidence of these craters is visible on Earth or Venus today. So, if Venus had a moon and that moon crashed into Venus thousands of millions of years ago, then the impact crater would likely have been erased by now.

And if Venus got hit by a sufficiently large object, then the energy of the impact might melt the entire surface of Venus, so that no lasting crater can form.

So, absence of very big craters from Venus is not evidence that Venus never had big moons.