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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)

This problem occurs when react native run Android

Solutions: 1. Delete the node in the project_ modules

2. Re execute NPM install

If the simulator has the above problems, if the solution is not good, delete the simulator in Android studio and re create your virtual devices