Anyone can, so it is hard to know who first concluded the Earth wasn’t flat.
For one thing, it depends on scale. Locally, of course, we all know it isn’t flat. There are hills and valleys, mountains and seas. So that means one needs to consider distances larger than tens of miles or kilometers - distances such that the variation from being flat due to the terrain is insignificant. But at distances of tens of miles or kilometers, is sufficient to recognize that it isn’t flat on that scale either if one recognizes what a horizon is.
There are places on Earth where it seems to be flat for large distances. Here are some of those:
But in each of those places, there is a well-defined horizon line - meaning, you cannot see beyond it. Why? Because the Earth’s surface is everywhere curved (or convex). In that last image, for example, there are hills or mountains in the background, but you cannot see their bases (especially those much farther away near the right side of the photo). In the middle picture, about fifty or a hundred miles beyond that horizon line, there are mountains which you cannot see. In the photo of the Pacific Ocean - taken from only a few feet above the level of the sea, one can only see a few miles out into the ocean, even though we know it goes for thousands.
So who first noticed that? We have no idea. But as far as we know, Eratosthenes with the first to actually take measurements that allowed calculating the radius of curvature. But that means people long before him already knew it was curved, they just hadn’t measured the curvature.