CREATE REACT NATIVE APPをNPMでインストールを行う際にエラーが発生する。

前提・実現したいこと

react native packcageを実行したい

発生している問題・エラーメッセージ

桜マークのWELCOME TO　REACT NATIVEが出た際、
To reload the app press "r"
To open developer menu press "d"　

→npm run android、npm run ios　を実行。

その後アプリケーションの作成・実行しても上手く行かず。
初心者の質問で大変恐縮ですが分かる方いらっしゃいましたら教えて下さい。

> @ start /Users/user/my-react-app/my-react-app
> react-native start

                                                          
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                 Welcome to React Native!
                Learn once, write anywhere



To reload the app press "r"
To open developer menu press "d"

warn No apps connected. Sending "reload" to all React Native apps failed. Make sure your app is running in the simulator or on a phone connected via USB.
info Reloading app...
warn No apps connected. Sending "reload" to all React Native apps failed. Make sure your app is running in the simulator or on a phone connected via USB.
info Reloading app...
warn No apps connected. Sending "devMenu" to all React Native apps failed. Make sure your app is running in the simulator or on a phone connected via USB.
info Opening developer menu...


shimoda-no-MacBook-Air:my-react-app user$ npm run android

> @ android /Users/user/my-react-app/my-react-app
> react-native run-android

info Running jetifier to migrate libraries to AndroidX. You can disable it using "--no-jetifier" flag.
Jetifier found 1277 file(s) to forward-jetify. Using 4 workers...
info Starting JS server...
/bin/sh: adb: command not found
info Launching emulator...
error Failed to launch emulator. Reason: No emulators found as an output of `emulator -list-avds`.
warn Please launch an emulator manually or connect a device. Otherwise app may fail to launch.
info Installing the app...
No Java runtime present, requesting install.

error Failed to install the app. Make sure you have the Android development environment set up: https://reactnative.dev/docs/environment-setup. Run CLI with --verbose flag for more details.
Error: Command failed: ./gradlew app:installDebug -PreactNativeDevServerPort=8081
No Java runtime present, requesting install.
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @ android: `react-native run-android`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @ android script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/user/.npm/_logs/2020-08-11T09_39_44_369Z-debug.log
shimoda-no-MacBook-Air:my-react-app user$ npm run ios

> @ ios /Users/user/my-react-app/my-react-app
> react-native run-ios

error Could not find "Podfile.lock" at /Users/user/my-react-app/my-react-app/ios/Podfile.lock. Did you run "pod install" in iOS directory?
error Could not find the following native modules: RNGestureHandler, RNReanimated, RNScreens. Did you forget to run "pod install" ?
info Found Xcode project "myreactapp.xcodeproj"
xcrun: error: unable to find utility "simctl", not a developer tool or in PATH
error Could not get the simulator list from Xcode. Please open Xcode and try running project directly from there to resolve the remaining issues. Run CLI with --verbose flag for more details.
Error: Command failed: xcrun simctl list --json devices
xcrun: error: unable to find utility "simctl", not a developer tool or in PATH
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @ ios: `react-native run-ios`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @ ios script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/user/.npm/_logs/2020-08-11T09_40_00_683Z-debug.log
shimoda-no-MacBook-Air:my-react-app user$ a
-bash: a: command not found
shimoda-no-MacBook-Air:my-react-app user$