React-native run-android does'nt work

FAILURE: Build failed with an exception.

  • What went wrong:
    Execution failed for task ‘:app:installDebug’.

com.android.builder.testing.api.DeviceException: No connected devices!

  • Try:
    Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

  • Get more help at https://help.gradle.org

BUILD FAILED in 1s
28 actionable tasks: 1 executed, 27 up-to-date
Could not install the app on the device, read the error above for details.
Make sure you have an Android emulator running or a device connected and have
set up your Android development environment:
https://facebook.github.io/react-native/docs/getting-started.html
events.js:167
throw er; // Unhandled ‘error’ event
^

Error: spawn cmd.exe ENOENT
at Process.ChildProcess._handle.onexit (internal/child_process.js:238:19)
at onErrorNT (internal/child_process.js:413:16)
at process.internalTickCallback (internal/process/next_tick.js:72:19)
Emitted ‘error’ event at:
at Process.ChildProcess._handle.onexit (internal/child_process.js:244:12)
at onErrorNT (internal/child_process.js:413:16)
at process.internalTickCallback (internal/process/next_tick.js:72:19)

when i run the react-native run-android command i got these lines of errors why?