Just guessing here, but maybe the spaces in the path could create a problem. Other than that, I know the “Standard Assets” folder is somewhat sealed and hard to reach from other folders and such. You might want to try placing it in another folder.
I got the error when I deleted the splat map from the terrain from the asset folder, not triggered by one of my scripts but by unity itself.
I found the error.
To load a resource at runtime, it seems it must be placed in a folder(or subfolder to) called “resources”.
I found this, and I now have GoodDirt.
The errors about terrain splats are because you have a terrain with empty texture references Select your terrain, and make sure all the entries defined in the “Paint” tab are assigned to valid textures.