I want to use watermark with this script

-- Access the Lightroom SDK namespaces.
local LrDialogs = import 'LrDialogs'
local LrFunctionContext = import 'LrFunctionContext'
local LrBinding = import 'LrBinding'
local LrView = import 'LrView'

local LrApplication = import 'LrApplication'
local LrExportSession = import 'LrExportSession'
local LrTasks = import 'LrTasks'

-- Process pictures and save them as JPEG
local function processPhotos(photos, outputFolder)
LrFunctionContext.callWithContext("export", function(exportContext)

local progressScope = LrDialogs.showModalProgressDialog({
title = "Auto applying presets",
caption = "",
cannotCancel = false,
functionContext = exportContext
})

local exportSession = LrExportSession({
photosToExport = photos,
exportSettings = {
LR_collisionHandling = "rename",
LR_export_bitDepth = "8",
LR_export_colorSpace = "sRGB",
LR_export_destinationPathPrefix = outputFolder,
LR_export_destinationType = "specificFolder",
LR_export_useSubfolder = false,
LR_format = "JPEG",
LR_jpeg_quality = 1,
LR_minimizeEmbeddedMetadata = true,
LR_outputSharpeningOn = false,
LR_reimportExportedPhoto = false,
LR_renamingTokensOn = true,
-- LR_size_doConstrain = true,
LR_size_doNotEnlarge = true,
-- LR_size_maxHeight = 2000,
-- LR_size_maxWidth = 2000,
-- LR_size_resolution = 72,
LR_size_units = "pixels",
LR_tokens = "{{image_name}}",
LR_useWatermark = true,
LR_watermarking_id = "{{simpleCopyrightWatermark}}",
}
})

local numPhotos = exportSession:countRenditions()

local renditionParams = {
progressScope = progressScope,
renderProgressPortion = 1,
stopIfCanceled = true,
}

for i, rendition in exportSession:renditions(renditionParams) do

-- Stop processing if the cancel button has been pressed
if progressScope:isCanceled() then
break
end

-- Common caption for progress bar
local progressCaption = rendition.photo:getFormattedMetadata("fileName") .. " (" .. i .. "/" .. numPhotos .. ")"

progressScope:setPortionComplete(i - 1, numPhotos)
progressScope:setCaption("Processing " .. progressCaption)

rendition:waitForRender()
end
end)
end

-- Import pictures from folder where the rating is not 2 stars 
local function importFolder(LrCatalog, folder, outputFolder)
local presetFolders = LrApplication.developPresetFolders()
local presetFolder = presetFolders[1]
local presets = presetFolder:getDevelopPresets()
LrTasks.startAsyncTask(function()
local photos = folder:getPhotos()
local export = {}

for _, photo in pairs(photos) do
if (photo:getRawMetadata("rating") ~= 2 ) then
LrCatalog:withWriteAccessDo("Apply Preset", function(context)
for _, preset in pairs(presets) do
photo:applyDevelopPreset(preset)
end
photo:setRawMetadata("rating", 2)
table.insert(export, photo)
end)
end
end

if #export > 0 then
processPhotos(export, outputFolder)
end
end)
end

-- GUI specification
local function customPicker()
LrFunctionContext.callWithContext("showCustomDialogWithObserver", function(context)

local props = LrBinding.makePropertyTable(context)
local f = LrView.osFactory()

local outputFolderField = f:edit_field {
immediate = true,
value = "D:\\Pictures"
}

local staticTextValue = f:static_text {
title = "Not started",
}

local function myCalledFunction()
staticTextValue.title = props.myObservedString
end

LrTasks.startAsyncTask(function()

local LrCatalog = LrApplication.activeCatalog()
local catalogFolders = LrCatalog:getFolders()
local folderCombo = {}
local folderIndex = {}
for i, folder in pairs(catalogFolders) do
folderCombo[i] = folder:getName()
folderIndex[folder:getName()] = i
end

local folderField = f:combo_box {
items = folderCombo
}

local watcherRunning = false

-- Watcher, executes function and then sleeps 60 seconds using PowerShell
local function watch()
LrTasks.startAsyncTask(function()
while watcherRunning do
LrDialogs.showBezel("Processing images.")
importFolder(LrCatalog, catalogFolders[folderIndex[folderField.value]], outputFolderField.value)
if LrTasks.canYield() then
LrTasks.yield()
end
LrTasks.execute("powershell Start-Sleep -Seconds 3")
end
end)
end

props:addObserver("myObservedString", myCalledFunction)

local c = f:column {
spacing = f:dialog_spacing(),
f:row {
fill_horizontal = 1,
f:static_text {
alignment = "right",
width = LrView.share "label_width",
title = "Watcher running: "
},
staticTextValue,
},
f:row {
f:static_text {
alignment = "right",
width = LrView.share "label_width",
title = "Select folder: "
},
folderField
},
f:row {
f:static_text {
alignment = "right",
width = LrView.share "label_width",
title = "Output folder: "
},
outputFolderField
},
f:row {
f:push_button {
title = "Process once",

action = function()
if folderField.value ~= "" then
props.myObservedString = "Processed once"
importFolder(LrCatalog, catalogFolders[folderIndex[folderField.value]], outputFolderField.value)
else
LrDialogs.message("Please select an input folder")
end
end
},
f:push_button {
title = "Watch every 3s",

action = function()
watcherRunning = true
if folderField.value ~= "" then
props.myObservedString = "Running"
watch()
else
LrDialogs.message("Please select an input folder")
end
end
},
f:push_button {
title = "Pause watcher",

action = function()
watcherRunning = false
props.myObservedString = "Stopped after running"
end
}
},
}

LrDialogs.presentModalDialog {
title = "Auto Export fullsize Watcher",
contents = c,
-- Preferrably cancel should stop the script 
-- OK can be changed to run in background
-- actionBinding = {
-- enabled = {
-- bind_to_object = props,
-- key = 'actionDisabled'
-- },
-- }    
}

end)

end)
end

customPicker()

i want to add watermark to this script, but always fail, maybe whats wrong with my code??