Did you make your own Heightmap/TerrainMask. Or is it that one map in the Beginning that is making an Error message appear?
-Upon lack of Information I can't help that much-
If however you did make your own Heightmap:
Make sure it is .png Format.
513x513
Greyscale
Saved in Computer>LocalDisk>FeralHeart>Media>Terrains
If it is a TerrainMask:
.png Format
513x513
RGB
Saved in that position aswell.
If neither of those are what you need then check this.
Computer>LocalDisk>FeralHeart>Media>Terrains>default
Make sure the Terrain Mask & Heightmap are still there.