Script works perfectly in client, but not server..?

This script works fine is client, but not server.. its in a server script also.

I get this error in my developer console : attempt to index field 'LocalPlayer' (a nil value)

local player = game.Players.LocalPlayer
local cash = game.Players.LocalPlayer.leaderstats
enable = true

function CompletePurchase()
    if enable == true then
    enable = false
end
   cash.Cash.Value = cash.Cash.Value + 10
end

local MarketplaceService = game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 23429504
enable = true

MarketplaceService.ProcessReceipt = function(receiptInfo) 


    for i, player in ipairs(game.Players:GetChildren()) do
        if player.userId == receiptInfo.PlayerId then


            if receiptInfo.ProductId == productId then

                CompletePurchase()
            end
        end
    end 

    local playerProductKey = "plr_" .. receiptInfo.PlayerId .. "_pur_" .. receiptInfo.PurchaseId
    ds:IncrementAsync(playerProductKey, 1)  

    return Enum.ProductPurchaseDecision.PurchaseGranted     
end