Unfortunately, it seems that the new 2018.3 Unity is having trouble again. Simply setting the Subset to 2.0 no longer helps, "Ports" is no longer defined. I was able to have Ports recognized by downloading it from NuGet and adding the .dll to the Plugins folder in Unity. However, when I go to run, it gives the error "PlatformNotSupportedError: System.IO.Ports is currently only supported on Windows". I am running this on the Editor on a Windows computer.