@OnlyIn delete it? Or do I have to add something?

yes, i would also recommend you to follow the Java Naming Conventions

the error says : Description    Resource    Path    Location    Type Return type for the method is missing    SkySkeletonRenderer.java   

How could i solve it ?

It says the error is this:

public SkySkeletonRenderer(EntityRendererManager renderManagerIn) {

 

show more of your code 1 line doesn't help, since this line looks good for me
also do you know basic java since the error should says whats wrong

renderer :

package net.mcreator.additionaladventure.entity.renderer;

import net.minecraftforge.fml.client.registry.RenderingRegistry;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import net.minecraftforge.client.event.ModelRegistryEvent;

import net.minecraft.util.math.MathHelper;
import net.minecraft.util.ResourceLocation;
import net.minecraft.entity.Entity;
import net.minecraft.client.renderer.model.ModelRenderer;
import net.minecraft.client.renderer.entity.model.EntityModel;
import net.minecraft.client.renderer.entity.layers.HeldItemLayer;
import net.minecraft.client.renderer.entity.MobRenderer;
import net.minecraft.client.renderer.entity.EntityRendererManager;

import net.mcreator.additionaladventure.entity.SkySkeletonEntity;

import com.mojang.blaze3d.vertex.IVertexBuilder;
import com.mojang.blaze3d.matrix.MatrixStack;

public class SkySkeletonRenderer {
	public static class ModelRegisterHandler {
		@SubscribeEvent
		public void registerModels(ModelRegistryEvent event) {
			RenderingRegistry.registerEntityRenderingHandler(SkySkeletonEntity.entity, renderManager -> {
				return new MobRenderer(renderManager, new Modelcustom_model(), 0.5f) {
					@Override
					public ResourceLocation getEntityTexture(Entity entity) {
						return new ResourceLocation("additional_adventure:textures/esqueletodelcielo.png");
					}
						public SkySkeletonRenderer(EntityRendererManager renderManagerIn) {
						super(renderManagerIn, new Modelcustom_model<>, 0.5F);
						this.addLayer(new HeldItemLayer(this));
					}
				};
			});
		}
	}

Error 1  : Description    Resource    Path    Location    Type
Cannot infer type argument(s) for <T> registerEntityRenderingHandler(EntityType<T>, IRenderFactory<? super T>)    SkySkeletonRenderer.java    /additional_adventure/src/main/java/net/mcreator/additionaladventure/entity/renderer    line 1    Java Problem

Error 2 : Description    Resource    Path    Location    Type
Constructor call must be the first statement in a constructor    SkySkeletonRenderer.java    /additional_adventure/src/main/java/net/mcreator/additionaladventure/entity/renderer    line 1    Java Problem
 

Error 3 :Description    Resource    Path    Location    Type
Syntax error, insert "( )" to complete Expression    SkySkeletonRenderer.java    /additional_adventure/src/main/java/net/mcreator/additionaladventure/entity/renderer    line 32    Java Problem

Please help me fix this.

did you use MCreator?

If a little of what I did there, I got it from MCreator

so you don't use MCreator for modding?

 

I don't use it so much

MCreator is not supported on this Forum

Ok thanks anyway for helping