When I debug build and run the app in iPhone Simulator, sometimes the packager doesn't run automatically for some unknown reason. I have to run the packager manually.
$ <YourWorkspace>/node_modules/react-native/scripts/launchPackager.command
I've got this issue with React Native 0.61.5 and 0.63.4 + macOS 10.15.7 + XCode 12.3.