Just now, TheRPGAdventurer said:

but my structure is not a big ass dungeon, it will not be found easily because miners in survival can't see it in caves that's why It needs a cave.

Then just check if the block at your x, y, z variables is air. Because then the origin is in a cave of some sort, but will not require 7x3x7 space.

2 hours ago, Animefan8888 said:

Then just check if the block at your x, y, z variables is air. Because then the origin is in a cave of some sort, but will not require 7x3x7 space.

can you please make it easy or show me how you edit the code?

2 minutes ago, TheRPGAdventurer said:

can you please make it easy or show me how you edit the code?

if block at x y z == AIR then
generate

That is psuedo-code not actual java code.

Then look for ONE air block. 

2 hours ago, Animefan8888 said:

if block at x y z == AIR then
generate

That is psuedo-code not actual java code.

I know it's just a pseudo code I mean how to implement it, Difficulty in understanding code remember.

if(world.getBlockState(new BlockPos(x, y, z)).getBlock() == Blocks.AIR) {
	dragonNest.generate(world, random, new BlockPos(x,y,z));
}

Should be about right...

19 hours ago, Kokkie said:

if(world.getBlockState(new BlockPos(x, y, z)).getBlock() == Blocks.AIR) {
	dragonNest.generate(world, random, new BlockPos(x,y,z));
}

Should be about right...

public void generateNestUnderground(World world, Random random, int chunkX, int chunkZ, Random rand) {        
        int x = chunkX * 16 + random.nextInt(16);
        int z = chunkZ * 16 + random.nextInt(16);
        int xy = x >> 4;
        int zy = z >> 4;
        int height = world.getChunkFromChunkCoords(xy, zy).getHeight(new BlockPos(x & 15, 0, z & 15));
        int y = height - random.nextInt(55); //30;
    
        if(world.getBiomeForCoordsBody(new BlockPos(x,y,z)).getBiomeClass().equals(BiomePlains.class)) {
            if((random.nextInt(10) + 1) <= 1) {
                                        
                if(world.getBlockState(new BlockPos(x, y, z)).getBlock() == Blocks.AIR) {
                     dragonNest.generate(world, random, new BlockPos(x,y,z));
                    
                   }
              }
         }
    }

update what do you think is wrong? when I add the method to spawn caves you guys suggested it will not work but if I remove it, it randomly spawns underground.

Well, the problem here is that you blindly pick a block below the surface. If that block isn't air, you don't generate.