The Earth might be flat. For example, in the game GTA, the world appears to be 3-dimensional, but my computer screen, with its pixels, only has two dimensions.

There's no way to be certain that the Earth is round.

Visually, the earth appears to be round.

But 2 dimensions are enough to simulate 3 dimensions.

For example, your computer screen, the pixels, it's all 2D.

We don't know the geometry of the universe. So, there's no way to know for sure if the earth is flat or round.

Obviously - the earth LOOKS round. But it's possible that it's flat.