【React Native】Execution failed for task ':app:installDebug'. > com.android.builder.testing.api.DeviceException: No online devices found.

error Failed to install the app. Make sure you have the Android development environment set up: https://facebook.github.io/react-native/docs/getting-started.html#android-development-environment. Run CLI with --verbose flag for more details.
Error: Command failed: ./gradlew app:installDebug -PreactNativeDevServerPort=8081

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:installDebug'.
> com.android.builder.testing.api.DeviceException: No online devices found.

* 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 5s

    at checkExecSyncError (child_process.js:611:11)
    at execFileSync (child_process.js:629:15)
    at runOnAllDevices (/Users/apple/Desktop/FireReactNaitve/node_modules/@react-native-community/cli-platform-android/build/commands/runAndroid/runOnAllDevices.js:94:39)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at async Command.handleAction (/Users/apple/Desktop/FireReactNaitve/node_modules/react-native/node_modules/@react-native-community/cli/build/index.js:164:9)

　　该问题出现在react-native run-android时。

　　解决办法：1、删除项目中的node_modules

　　　　　　　2、重新执行npm install

　　模拟器出现以上问题，如果办法不行，在Android Studio中删除掉模拟器，重新创建Your Vitual Devices即可。