In a recent update to Minecraft, Hardened Clay was renamed to Terracotta. However, Chisel has yet to adopt the name change in their code/localisation/texture file names. Also, there has yet to be coloured variants of terracotta added to Chisel.

As a side note. Forge still has an ore dictionary reference on terracotta called hardenedClay. This should now be called blockTerracotta and blockTerracottaColorless. Though I imagine it is up to Forge to make this change and not yourselves.

P.S. I would've done a PR for this. However, I couldn't get the environment setup and had no idea what branch was being used for 1.12 😛