https://pastebin.com/U3de2dp8

Caused by: net.minecraft.client.renderer.block.model.ModelBlockDefinition$MissingVariantException

Minecraft: 1.12.2
Forge: 14.23.1.2554
Chisel: 0.0.14.18

Solutions so far:

He could simply add an empty json and the error should be away.

If it does not need one, set the block to have no model like vanilla barriers do (It's one of the methods on Block)