Why the Survey Corps doesn't find a human body inside normal titan? Hange cut the neck of one titan to explore inside. Why couldn't she find out the main human body inside???

Maybe the main body was hidden quite deep so it was difficult to see. But if it was hidden quite deep, then how can Levi and other soldiers cut down the neck and kill the titan.

Is this a logical flaw in AOT?
The titans you are talking about are generic titans, which are different from Shape-shifting titans. Generic titans transformed permanently into titans. Their human bodies no longer exist. They can't transform back into humans. If you cut their nape, they are dead, once and for all.

This is different than titan shifters such Eren, because when they transform, they build a titan body around their human body. The human body acts as a center of control for the titan body. They can power down, whenever they want. They have inherited the Titan shifting power. That's why, if you cut the nape of a shifter, you'll find the human sitting there. And until you hurt/kill that human, they can transform again.