Many people come to report a bug about superflat not working, or even the amplified or large biomes world types. There is nothing we can currently do about this. Please do not make a new issue about it, I will close it. Here is the copy/paste answer I have been giving:
Terralith is a datapack (yes, even the mod version - it is a datapack packaged as a mod). Due to this Minecraft bug, worldgen datapacks cannot support other world types, such as Large Biomes, Amplified, or in your case: Superflat. Although it is a pain, you will need to disable Terralith when making and using superflat worlds. This will be the case until either Mojang fixes the bug, or we spend a long time custom coding a mod to fix that.
What Minecraft version(s) does this apply to?
All
Project version
All
Mod or datapack version?
All
Describe the bug
Many people come to report a bug about superflat not working, or even the amplified or large biomes world types. There is nothing we can currently do about this. Please do not make a new issue about it, I will close it. Here is the copy/paste answer I have been giving:
Terralith is a datapack (yes, even the mod version - it is a datapack packaged as a mod). Due to this Minecraft bug, worldgen datapacks cannot support other world types, such as Large Biomes, Amplified, or in your case: Superflat. Although it is a pain, you will need to disable Terralith when making and using superflat worlds. This will be the case until either Mojang fixes the bug, or we spend a long time custom coding a mod to fix that.