I'm using string.sub, but my output says "Invalid Argument #1 to "sub". What do I do?

The string.sub() on its first parameter it needs a string value,The thing you inserted is a model,models are instances

local MarketplaceService = game:GetService("MarketplaceService")--The Service
    local Players = game:GetService("Players")--More Services

    local productID = 1083524533--Product ID

    local function promptPurchase()--The function
        local player = Players.LocalPlayer--The Player
        MarketplaceService:PromptProductPurchase(player, productID)--Prompt it
        MarketplaceService.PromptProductPurchaseFinished:connect(function()--When it's done
            local stage = script.Parent.Parent.Parent.Parent.Parent.Team--The Team(From Alvin_Blox)
            print(string.sub(stage.Name,6,7))
end)--end
    end--end

    script.Parent.MouseButton1Click:Connect(promptPurchase)--Connect it

The second parameter of it,it tells in where character to start

string.sub("MyString",3,7)

m(1)y(2)s(3) It means that the string.sub will start in the "S" character and it will be "String"

And the last parameter that is optional,it tells where to stop,in this case it is a seven it will be: "Strin"

The string.sub,is basically a string cutter.

Hope it helps!