If the Earth is round, is anything actually flat? Is a plateau a real thing? You could literally question just about anything in existence and have a legitimate argument without ever being completely right or wrong.
Some people especially the conspiracy theorist like Alex jones believe so. But if you ask about flat, just look at water. The principle is that water always find the shape of a flat either in a glass of water, lake or the sea. Water is always flat. Just think about us in the middle of a flat water and the continents are floating.
And then ask how can god make us flood when the earth is round?
The earth is spherical in shape and that is why the moon and sun are also in that shape, that is why we can also have daylight in Nigeria and it would be midnight in the US, because the sun cannot shine on a round surface and it will reach every part of the shape, like a circle for instance. But assuming the earth is flat in shape, the whole world will experience daylight at the same time and night at the same time too.
Water is not flat in glass, instead it has a concave meniscus shape.