Hey Fatalitu,
The Restock hack is available for Minecraft 1.18.2 - 1.21.1 (Wurst 7.32 and later). Since you're using Wurst 6 for Minecraft 1.8, which is no longer receiving updates, unfortunately you won't be able to use this feature unless you update to Wurst 7 and a newer version of Minecraft.
If upgrading your Minecraft version isn't an option for you and you're planning on staying with 1.8, you might have to manually replenish your cactus supply in your hand the traditional way.
