First and foremost I'll take the chance to remind you that the earth is spherical in shape almost round like a football. But where would you say is the center of of a football. The center of it wouldn't be on the body or shield, it will be correct to say it is the indefinite space or middle inside the foot ball. That is the center of it.
Likewise when it comes to declaring where the center of the earth. A lot of claims have being raised naming one of these long list of places as the center of the earth, from Jerusalem to Mecca, Babylon or some place in the Himalayas to be centre of the Earth, even adding Israel, Saudi Arabia, Iraq and Nepal to the list.
The earth is a floating circle, the middle of it would be the perfect center of the earth. Think about it, if a circle was drawn on a flat surface, it will be easy to decide or declare a point in the middle of that circle as it's center. The center of an object is a point in some sense in the middle of the object (this includes the earth and a circle). Take for instance;
Take in another instance, the earth is a flat surface, here it will be easy to identify where the center lies, it could be depicted easily on any atlas map to be located as the popular null Island. Here is another example from a flat surfaced map;
It should be obvious bat this point that if I stand here in Africa trying bro stare directly at the center of the earth. I would only look straight towards the curved part of the earth, which makes me see the sky meeting with land as the end of my sight. Assuming I could see past that point, I would be staring not continuously from the curved part where my sight ended but directly into the universe.
Now, the center of the earth being spherical, a shape that doesn't stop turning, floating on magnetic waves has its center at what is scientifically called the "Earth Core".
This should be evident to let you know that the center or middle of the earth isn't on the surface. But within the inner space of the planet. To better understand this topic, I have share a video below. Do have a nice time exploring.