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)