Dev Product function expects code to use "end" when using "return"?

You had one too many ends. You should indent your code so you can see if you have the correct amount of ends. Your code should look like this:

local link = game:GetService("MarketplaceService")
deb = 0


script.Parent.ClickDetector.MouseClick:Connect(function(plr)
	local marketId = 377473848 --ID OF YOUR DEVELOPER PRODUCT  
	if deb == 0 then
    		deb = 1
   		link:PromptProductPurchase(plr,marketId)
    		link.ProcessReceipt = function(receiptInfo)
    		if plr.UserId == receiptInfo.PlayerId then
       			if receiptInfo.ProductId == marketId then
       				local megalodon = game:GetService("ServerStorage").Megalodon:Clone()
        			megalodon.Parent = game.Workspace
        			megalodon:MakeJoints()
        			megalodon:MoveTo(workspace.SharkPoint2.Position)
       				 wait(1)
   				 deb = 0
    				return Enum.ProductPurchaseDecision.PurchaseGranted
			end
		end
	end
end)