Apple has released the sixth beta of watchOS 5.3 to the developers

Release Notes:

General
Known Issues
Devices running watchOS 5.3 beta don’t appear as available for development in Xcode 10.2.1. (50638444)

Workaround: Run the following command in Terminal:

defaults write com.apple.dt.Xcode Enable43515398 -bool YES

  watchOS 5.3 Beta 6 released