I have a Mac early 2011 running MacOS high sierra 10.13.6. My Mac can\'t upgrade to higher MacOS versions.
I need to install XCode 11 to take advantage of SwiftUI.
Upgrade old macbook pro using this patch http://dosdude1.com/mojave/. You can easily use Xcode 11.
From Xcode 11 Release Notes:
Xcode 11 requires a Mac running macOS 10.14.3 or later.
So you cannot install Xcode 11 on macOS 10.13.6