and What did I do wrong since there is a red underline at code line 10?
game.ReplicatedStorage.CommandRemotes.ShowAdminNotification.OnClientEvent:connect(function()
errorDesc = script.parent.Description
errorDesc.Text = newPlayer.Name.. " has joined the server!"
--// Spaceing
script.Parent.Description.Text = errorDesc
script.Parent:TweenPosition(UDim2.new(0.8,0,0.8,0),Enum.EasingDirection.InOut,Enum.EasingStyle.Linear,0.2)
wait(10)
script.Parent:TweenPosition(UDim2.new(1,0,0.8,0),Enum.EasingDirection.InOut,Enum.EasingStyle.Linear,0.2)
end)

3 comments
So is this a Local script or a normal script? And please tell me what is it's parent.
Local script, frame
You can hover on the word that the red line is under and it will tell you the error.