Xcode 10.1 is the last version that runs on macOS High Sierra 10.13.6.
To download and work with Xcode on macOS High Sierra 10.13.6, you must use Xcode 10.1
Notes/assumptions: you want a version of Xcode that runs on macOS High Sierra 10.13.6 (released 2017). The latest Xcode does not support High Sierra; use Xcode 10.1 or Xcode 9.4.x (Xcode 10.1 is the last that officially supports 10.13.6).
Steps
Xcode 10.1 is the last version that runs on macOS High Sierra 10.13.6.
To download and work with Xcode on macOS High Sierra 10.13.6, you must use Xcode 10.1
Notes/assumptions: you want a version of Xcode that runs on macOS High Sierra 10.13.6 (released 2017). The latest Xcode does not support High Sierra; use Xcode 10.1 or Xcode 9.4.x (Xcode 10.1 is the last that officially supports 10.13.6).
Steps
