How do I fix the: attempt to index nil with 'PlayerId' error?

I have an error while trying to get the player/UserID when a dev product is purchased, here is my script:

local productID = 1270692852

local MPS = game:GetService('MarketplaceService')

local function processRecipet(RecieptInfo)

    local plr = game.Players:GetPlayerByUserId(RecieptInfo.PlayerId)

    if not plr  then
        -- plr probably left.
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    if RecieptInfo.PurchaseId == productID then
        if plr then
            local leaderstats = plr:WaitForChild('leaderstats')
            local Gems = leaderstats:WaitForChild('Gems')
            Gems.Value = Gems.Value + 50
        end
    end


    return Enum.ProductPurchaseDecision.PurchaseGranted

end

-- call back

MPS.ProcessReceipt = processRecipet()

The error is on this line:

    local plr = game.Players:GetPlayerByUserId(RecieptInfo.PlayerId)

All of this is in a server script.