The enhanced version of this script is available: https://gist.github.com/ruccho/2d1eb4aea3dfa55690c2ddc4419172ff

Generate Normal Map.lua

----------------------------------------------------------------------
-- Generate Normal Map
--
-- It works only for RGB color mode.
----------------------------------------------------------------------

if app.apiVersion < 1 then
    return app.alert("This script requires Aseprite v1.2.10-beta3")
  end
  
  local cel = app.activeCel
  if not cel then
    return app.alert("There is no active image")
  end

  local img = cel.image:clone()
  local position = cel.position
  
  if img.colorMode == ColorMode.RGB then
    local rgba = app.pixelColor.rgba
    local rgbaA = app.pixelColor.rgbaA
    for it in img:pixels() do
        local x = it.x
        local y = it.y
        local top = 2
        local left = 2
        local right = 2
        local bottom = 2
        if rgbaA(it()) < 255 then
            -- Transparent pixel to ignore
        else
        -- Pixel to prcess
            -- explore top
            if y > 0 then
                -- possible to explore
                local topPixel = img:getPixel(x, y - 1)
                if rgbaA(topPixel) < 255 then
                    top = 0
                elseif y > 1 then
                    topPixel = img:getPixel(x, y - 2)
                    if rgbaA(topPixel) < 255 then
                        top = 1
                    end
                end
            else
                top = 0
            end
            -- explore bottom
            if y < img.height - 1 then
                -- possible to explore
                local bottomPixel = img:getPixel(x, y + 1)
                if rgbaA(bottomPixel) < 255 then
                    bottom = 0
                elseif y < img.height - 2 then
                    bottomPixel = img:getPixel(x, y + 2)
                    if rgbaA(bottomPixel) < 255 then
                        bottom = 1
                    end
                end
            else
                bottom = 0
            end
            -- explore left
            if x > 0 then
                -- possible to explore
                local leftPixel = img:getPixel(x - 1, y)
                if rgbaA(leftPixel) < 255 then
                    left = 0
                elseif x > 1 then
                    leftPixel = img:getPixel(x - 2, y)
                    if rgbaA(leftPixel) < 255 then
                        left = 1
                    end
                end
            else
                left = 0
            end
            -- explore right
            if x < img.width - 1 then
                -- possible to explore
                local rightPixel = img:getPixel(x + 1, y)
                if rgbaA(rightPixel) < 255 then
                    right = 0
                elseif x < img.width - 2 then
                    rightPixel = img:getPixel(x + 2, y)
                    if rgbaA(rightPixel) < 255 then
                        right = 1
                    end
                end
            else
                right = 0
            end
            local light = 0

            -- -2 ~ +2
            local y_digit = - top + bottom
            local y = y_digit * 32 + 128
            local x_digit = - right + left
            local x = x_digit * 32 + 128
            local z_digit = math.max(math.abs(x_digit), math.abs(y_digit))
            local z = z_digit * -32 + 255
            local color = rgba(x,y,z,255)
            it(color)
        end
    end
    
  elseif img.colorMode == ColorMode.GRAY then
    return app.alert("This script is only for RGB Color Mode")
  elseif img.colorMode == ColorMode.INDEXED then
    return app.alert("This script is only for RGB Color Mode")
  end

  local sprite = app.activeSprite
  local frame = app.activeFrame
  local currentLayer = app.activeLayer
  local newLayerName = currentLayer.name .. "_NormalGenerated"
  local newLayer = nil
  for i,layer in ipairs(sprite.layers) do
    if layer.name == newLayerName then
        -- the layer to write normal on is already exists
        newLayer = layer
    end
  end
  if newLayer == nil then
    newLayer = sprite:newLayer()
    newLayer.name = newLayerName
  end
  local newCel = sprite:newCel(newLayer, frame, img, position)

  app.refresh()

I have the latest aseprite version, but i get this attempt to compare nil with number at line 7

commenting out that part, a out of bounds error shows at line 128

Works in 1.2.12.1. But requires baked layer, doesn't work with Groups.
Also script is not for proper normal map creation (from grayscale image of "depth" values), but rather treats each pixel as "all or nothing", "automatically" smoothing edges of a sprite.

Is it possible for you to add an invert Y option?

Is it possible for you to add an invert Y option?

just add a - before the y in line 102 :)