Hello
I'm reporting a bug because I've created a preset to make a flat world since version 1.16.3, but there's been an error.
I attached the bug report, so please refer to it.

I think you threw an exception because you didn't enter the baiom, but it would be better to redden the text box or use pop-up lights to guide the player.
(java.lang.IllegalStateException: Missing: ResourceKey[minecraft:worldgen/biome / minecraft:])

(I used machine translation to translate the contents. Please understand even if the content is not natural.)