You will need an Android device to run your React Native Android app. This can be either a physical Android device, or more commonly, you can use an Android Virtual Device which allows you to emulate an Android device on your computer.
Either way, you will need to prepare the device to run Android apps for development.
Using a physical device
If you have a physical Android device, you can use it for development in place of an AVD by plugging it in to your computer using a USB cable and following the instructions here.
Using a virtual device
If you use Android Studio to open ./AwesomeProject/android, you can see the list of available Android Virtual Devices (AVDs) by opening the “AVD Manager” from within Android Studio.
If you have just installed Android Studio, you will likely need to create a new AVD. Select “Create Virtual Device…”, then pick any Phone from the list and click “Next”, then select the Pie API Level