I'm using Windows 10 OS and Unity 2018.3.7f1. The target platform just shows Windows. I've tried downloaded the linux target package, but once I import it, the Unity Editor claims that I've already has all the packages installed. Another weird stuff it's that isn't shown the "switch platform" buttom, although it has been seen even in olders Unity version, over the Internet forums and videos. Am I forget something? I have the feeling that once you create a project for a SO, you can't aim another platform. Am I correct?