nom start が作動しない

Question

ソースコード

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

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...
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...
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...

接続アプリがないと書かれてますがこの場合の接続アプリとはなんでしょうか

Answer 1

Make sure your app is running in the simulator or on a phone connected via USB.

ということで，

• PC上のシミュレーターでアプリが起動している
• USBに接続された端末でアプリが起動している

のどちらかが求められています．

なので
aもしくはiを押すか，
または，

npm run android
npm run ios

して先にアプリを起動させる必要があります．