attempt to index local 'player' a nil value?

_G.Data = {
koins = 500,
level = 1,
rank = "Bronze",
print('make sure to add jersey'),
--Helmet = (link)
--Helmet.Name = "Helmet",
--local mouthp = Instance.new("MeshPart",Data),
--mouthp.Name = "Mouthpiece",
xp = 1
}

local DSS = game:GetService("DataStoreService")

local datastore = DSS:GetDataStore("YourChampion")


function generateDataKey(player)
        local ret = "cid_"..player.userId
        return ret
end


for index,value in pairs (_G.Data) do
    datastore:UpdateAsync(index, function() return value end) -- index will be our 
end 



function generateDataTable(player)
    local dataTable = {
        koins = player.PlayerGui.Nice.Data.koins.Value,
        level = player.PlayerGui.Nice.Data.Level.Value,
        xp = player.PlayerGui.Nice.Data.xp.Value,
        rank = player.PlayerGui.Nice.Data.Rank.Value
    }
    return dataTable
end

function saveDataForPlayer(player)
local Success, ErrorMsg = pcall(function()
  print('Saving Data') 
  local key = generateDataKey(player)
  local data = generateDataTable(player) 
  datastore:SetAsync(key, data) 
end)

if not Success then
warn(ErrorMsg) -- if it errors, it will provide a reason on why it errored 
end
    local key = generateDataKey(player)
    local data = generateDataTable(player)
    datastore:SetAsync(key, data)
end

function inputDataToPlayer(player, data)
    player.PlayerGui.Nice.Data.koins.Value = data.koins
    player.PlayerGui.Nice.Data.Level.Value = data.Level
    player.PlayerGui.Nice.Data.xp.Value = data.xp
    player.PlayerGui.Nice.Data.Rank.Value = data.Rank
end

function loadDataForPlayer(player)
    local key = generateDataKey(player)
    local data = datastore:GetAsync(key)
    inputDataToPlayer(player, data)
end

if _G.Data == nil then
    inputDataToPlayer()
else
    loadDataForPlayer()
end


game.Players.PlayerAdded:connect(function(player)
    loadDataForPlayer(player) --Load first thing when they join
                player.PlayerGui.Nice.Data.Rank.Changed:connect(function()
                    saveDataForPlayer(player)
                    end)
end)

game.Players.PlayerRemoving:connect(saveDataForPlayer()) --Save data when the player leaves the game
`Workspace.Data:19: attempt to index local 'player' (a nil value)`