Terrain prototype goes black when building project

Issue: When testing my game in Unity, everything works perfectly fine. It is only when I build the project and run it as an executable file, that my terrain texture turns completely black.

Other Information: I have done some testing and have tried not adding a texture so that in the editor the terrain is white. Surprisingly when I build it, the terrain is also the same colour (white).

 splatPrototypes[0] = new SplatPrototype();
          //splatPrototypes[0].texture =  splat0;
          splatPrototypes[0].texture = (Texture2D)Resources.Load("TerrainTextures/UNIcolourGrass");
          splatPrototypes[0].tileSize = new Vector2( splatTileSize0,  splatTileSize0);
         
          splatPrototypes[1] = new SplatPrototype();
          //splatPrototypes[1].texture =  splat1;
          splatPrototypes[1].texture = (Texture2D)Resources.Load("TerrainTextures/UNIcolourGrass");
          splatPrototypes[1].tileSize = new Vector2( splatTileSize1,  splatTileSize1);

Any help would be very much appreciated as I am completely stumped!