Why wont these dev products work?

You didn't define player.

Whenever you see that kind of error, remember that it means you have to define it. It doesn't know what plr is

Here put this code in a script then put the script in StarterPack:

local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")

CASHID = 00000000 -- Put the Developer Product ID here and erase the 0's

MarketplaceService.ProcessReceipt = function(receiptInfo)
    local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_product_" .. receiptInfo.ProductId
    local numberBought = ds:IncrementAsync(playerProductKey, 1)
    for i,v in pairs (game.Players:GetChildren()) do
        if v.userId == receiptInfo.PlayerId then
            if receiptInfo.ProductId == CASHID then


                lds = v:FindFirstChild("leaderstats")
                if lds ~= nil then
                    cs = lds:FindFirstChild("Credits") --Change to your leaderstat
                    if cs ~= nil then
                        cs.Value = cs.Value + 250 --Change to the amount you want this to add
                    end
                end
            end
        end
    end
    return Enum.ProductPurchaseDecision.PurchaseGranted     
end

Don't forget to say this answered you and give it a thumbs up because I am working for a good reputation!

On line 16 change plr to player. Plr is nil to the script since it was never defined.

And add to line 19 or else the player will get credits on entry. return Enum.ProductPurchaseDecision.PurchaseGranted

Please refrain from reposting this question.

Any errors, just comment on this answer.

-- setup local variables
local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 20768662

-- define function that will be called when purchase finished
MarketplaceService.ProcessReceipt = function(receiptInfo) 

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

            -- check which product was purchased
            if receiptInfo.ProductId == productId then
                --handle purchase. In this case we are highering the player's Credits value         
                    player.leaderstats.Credits.Value = plr.leaderstats.Credits.Value + 250
                -- more feedback for the player.
                game.Workspace.DisplayScreen.SurfaceGui.TextBox.Text = player.Name .. " has purchased more credits."
return Enum.ProductPurchaseDecision.PurchaseGranted --this is line 19.
                    end
                end
            end
        end