title: Emulator is not working: Execution failed for task ':app:installDebug'
body: <p>First I'm a newbie in react-native. I start a React-native in Facebook official site react-native tutorial. When I follow the react-native android setup step by step, then I run this project When a show this message please Resolve this Error, and Suggest me. Please help me.</p>
  
  <pre><code>D:\Linux\IDE\All-Workspace\JavaScript-Workspace\React-Workshop\AwesomeProject&gt;react-native run-android
  JS server already running.
  Building and installing the app on the device (cd android &amp;&amp; gradlew.bat install debug)...
  
  FAILURE: Build failed with an exception.
  
  * What went wrong:
  Execution failed for task ':app:installDebug'.
  &gt; 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 6s
  27 actionable tasks: 1 executed, 26 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.
  </code></pre>