The error means that the terrainTypes list doesn't contain the correct amount of elements. Each vertex must get one set of terrainTypes coordinates. Whenever something is added to the vertices list, something must also be added exactly once to the terrainTypes list. You have a mismatch somewhere, which is easy to overlook. I suggest you follow the entire tutorial as skipping bits can come back to haunt you later in a long series like Hex Map. You can make your own adjustments in a parallel project. That gives you a better idea of what's different.