When i get a grainy looking image, the first thing i'll check is the sunlight and it's setting in the top right corner called "Softness". Even a softness setting of 5.00° gives me already a grainy look, so i usually make sure, the value is set at 0.00° for a clear render.

Or there could be trouble in calculating with a texture, that is scaled too small compared to the size of the object/terrain.
Well, at least that's what i believe could be two reasons for a grainy result ...