TerrainData is missing splat texture (0,1,2,3)

I would want to add my personal experience. @ChaoticCherryCake is a good answer but still I will explain what happened to me.

I had this error message : "TerrainData is missing splat texture 6". So I select my Terrain and go in the "Paint Texture" tab (with the brush icon). I notice I have 6 textures. (first image below)

But as a programmer, I think maybe the error message is counting textures from 0. So the 6th texture would be at index 5. I click where my 7th texture (index 6) should be... (second image) Bingo. The texture was here but invisible. What a weird bug !! Still don't know how it happened, but I'm glad to see it fixed. Hope it will help you guys :)

It may be a bit late, but I think the terrainData can't find the textures. So I think you need to reassign the textures of the terrain.

Hello sorry for my bad english language . i solved this: for example if u choose this texture "GrassHillAlbedo" search for it in your project>Standard Assets>Environment files in unity , then click on it , then in inspector window under the advanced option for "GrassHillAlbedo" u can find a check box that says "Read/Write Enable" mark it and then apply in the bottom . make sure u are not in play mode when you are doing this . and remove your texture and add it again then start play mode it should worked.

and for be more careful u can also change your whole default save project from Users>public>.... to a new location in your PC that don't be in public folder .