Fixing iPhone is busy: Preparing Apple Watch for development via iPhone

Enable Apple Watch's Airplane Mode to fix.

iPhone is busy: Preparing Apple Watch for development via iPhone
  1. Turn Airplane mode on your Apple Watch.
  2. Ensure that you deselected both Bluetooth and Wi-Fi at your Apple Watch Airplane Mode Behavior setting.
  3. Try rerunning Xcode Build.
  4. Once Xcode functions again, you can disable Airplane Mode on your Apple Watch.