Failed to call field 'insert' (a nil value). How can I fix this?

for i = 1,5 do
        but = Instance.new("TextButton",scr)
        but.Text = ""
        but.TextScaled = true
        but.Size = txt.Size
        but.Position = UDim2.new(1,0,1 - txt.Size.Y.Scale * (i + 1),0)
        but.Style = Enum.ButtonStyle.RobloxRoundDropdownButton
        table.insert(allbuttons,but)
end

idk what this is?