You know, the code thingy. In the DefaultBiomeFeatures you sent me to, each builder has a different p_ number thing, like p_243731_0_ or p_243734_0_. I didn't know if I needed one, but apparently event.getSpawns() works. The instructions about removing the bloodghastSpawns were unclear, but I think it should look something like this. You know, without the errors.
The underlined section in red is asking for an EntityClassification, and I gave it an EntityType. But I don't believe I have a classification to give it. I tried 'classification' and 'PlaneswalkerEntities.classification' but those didn't work.