Try using vb.net instead of C#. I know uipath has added c# functionality with some versions (but not all). I’m also not sure if you have to set that language somewhere within settings as i haven’t used it myself.
There are online calculators that do a decent job switching back and forth between vb.net and c# syntax - i usually use either of these 2: CodeTranslator: Code Translation From VB.NET <-> C# <-> TypeScript <-> Java or http://converter.telerik.com/
I’m guessing your example was purposefully simple, but just in case that is your real use case there isn’t a reason for using linq for something this simple. You can just use List.Sort instead 