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About Suppliers...https://www.geeksforgeeks.org/supplier-interface-in-java-with-examples/

The constructor of ItemStack is looking for an Item, but you has passed in an Item Supplier. You need to use RegistryObject#get to get the Item from the supplier, for example RegistryObject<Item> MY_ITEM = ..... new ItemStack(MY_ITEM.get());

Alright, let's try another way: If you extend the BiomeGenerationSettings.Builder class, and add your own method that takes a structure supplier, i.e. something like this: public static class Builder extends BiomeGenerationSettings.Builder { public BiomeGenerationSettings.Builder withStructureSupplier(Supplier<StructureFeature<?, ?>> structure) { this.structures.add(structure); return this; } } and then use this builder to create your biome instead. Does that work?