I am completely new to react-native and android. I have downloaded a project from Github and used commands yarn install and react-native run-android to
Just download from official template https://raw.githubusercontent.com/facebook/react-native/master/template/android/app/debug.keystore
Copy debug.keystore into your android/app/.
Ex:
mv ~/Downloads/debug.keystore /Users/lazaro/apps/myapp/android/app/
ref: https://github.com/facebook/react-native/issues/25629#issuecomment-513245590