Developer product textbutton that kills you when bought?

Hello, i'm trying to kill the player who buys a developer product. the script is not working below is the error in the output when i buy it in the game using F9, and the script i'm using.

Players.smalldaniel.PlayerGui.ScreenGui.TextButtontt.Script:15: attempt to index global 'MarketplaceService'(a nil value)

productId = 21073044 

MarketplaceService.ProcessReceipt = function(receiptInfo) 

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

            if receiptInfo.ProductId == productId then
        local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_purchase_" .. receiptInfo.PurchaseId
            ds:IncrementAsync(playerProductKey, 1)  
        return Enum.ProductPurchaseDecision.PurchaseGranted  
               end Workspace.player.Humanoid.Health = 0
            end
        end
end

Thank you