How can i fill a block with bricks?

You should store objects in ServerStorage and not in Lighting.

If you want to place the part somewhere within the boundaries of the box, you can multiply the block's CFrame by some offset to translate it.

block.CFrame = box.CFrame * CFrame.new(Vector3.new(math.random()*2-1, math.random()*2-1, math.random()*2-1)*27)

If you only want it to be placed in 6x6x6 cells within the box:

block.CFrame = box.CFrame * CFrame.new(math.random(-4,4)*6, math.random(-4,4)*6, math.random(-4,4)*6)

In order to fill the whole box you can just loop on each axis

for x=-4,4 do
	for y=-4,4 do
		for z=-4,4 do
			local new = block:Clone()
			new.CFrame = box.CFrame * CFrame.new(x*6, y*6, z*6)
			new.Parent = box.Parent
		end
	end
end