Although the book claims to be up-to-date, the way it registers items/blocks is already outdated. Please use DeferredRegister as oppose to RegistryEvent.

DeferredRegister: https://github.com/MinecraftForge/MinecraftForge/blob/1.16.x...

Also, I find Cadiboo's Example Mod super helpful: https://github.com/Cadiboo/Example-Mod