Possible Terrain Painting Bug...?

I'm extremely new to Unity so I could be doing something wrong. I'm just trying to paint this green texture onto the terrain but it won't paint. I've followed tutorials on how to do this but it won't work.

 

You terrain must use TerrainLit material. Use the Sand texture for the first layer. That will cover the whole terrain with sand. Use additional layers to paint other textures on the sand.