I don't understand this error, I've scoured the .jsons a bunch, I need a second pair of eyes.

java.util.concurrent.CompletionException: java.lang.IllegalStateException: Trying to access unbound value 'ResourceKey[minecraft:dimension / multiscape:underdark]' from registry Registry[ResourceKey[minecraft:root / minecraft:dimension] (Stable)]
15:13:59.309
Caused by: java.lang.IllegalStateException: Trying to access unbound value 'ResourceKey[minecraft:dimension / multiscape:underdark]' from registry Registry[ResourceKey[minecraft:root / minecraft:dimension] (Stable)]

I'm new to this and google is swamped with irrelevant results. Also looking for any tips for debugging. So far, I've been having grammar differences with every tutorial, even 1.18.2 tuts. I have the datapack tools extension on VSCode and the log open from the launcher.
Thanks all!