How do you properly get the DataStore?

local PurchaseHistory = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")

MarketplaceService.ProcessReceipt = function(receiptInfo) 
    local playerProductKey = receiptInfo.PlayerId .. ":" .. receiptInfo.PurchaseId
    if PurchaseHistory:GetAsync(playerProductKey) then
        return Enum.ProductPurchaseDecision.PurchaseGranted
    end

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

            if receiptInfo.ProductId == id then
            plr.leaderstats.Coins.Value = plr.leaderstats.Coins.Value +1000
            end
        end 
    end
    PurchaseHistory:SetAsync(playerProductKey, true)
    return Enum.ProductPurchaseDecision.PurchaseGranted     
end

I didn't code this, I've been messing around with a code from the wiki; http://wiki.roblox.com/index.php?title=API:Class/MarketplaceService

I know what it does and what it should do.

When I buy the DevProduct, it shows in console(f9) Cannot access DataStore from this client.

Does anyone know a solution?

Screenshot of error: http://prntscr.com/88yisp