Trees on terrain not placed on some areas

Hi,
I have a question on a a game I’m making. I wanted to have a flat shaded terrain which I’ve done but the problem is that when I place trees or some grass or anything like that, some areas of the terrain mesh don’t seem to “work”. Also in those areas my character falls to the abyss through the ground.
I’m assuming it has to do something with the normals because I’m using the shader graph to make the flat shader.

If anyone has any idea or suggestion please leave a comment.