Define location with an ANDROID_SDK_ROOT environment variable or by setting the sdk.dir

エラー

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:
Could not determine the dependencies of task ':app:installDebug'.
> SDK location not found. Define location with an ANDROID_SDK_ROOT environment variable or by setting the sdk.dir path in your project's local properties file at '/Users/user/your-app/android/local.properties'.

というWarningが発生し、React NativeのビルドがFAILEDになっていた。

解決策

.zshrcなどにANDROID_SDK_ROOTを定義するかlocal.propertiesファイルを作成し、sdk.dirを定義する。今回は、local.propertiesを作成する方で対応した。

touch /Users/user/your-app/android/local.properties

local.properties
## This file must *NOT* be checked into Version Control Systems,
# as it contains information specific to your local configuration.
#
# Location of the SDK. This is only used by Gradle.
# For customization when using a Version Control System, please read the
# header note.
#Thu Dec 10 16:26:54 JST 2020
sdk.dir=/Users/user/Library/Android/sdk

参考

android - Where is android_sdk_root? and how do I set it.? - Stack Overflow