Step 1:
Open the package.json file and add the following line
"build:ios": "react-native bundle --entry-file='index.js' --bundle-output='./ios/main.jsbundle' --dev=false --platform='ios'"
Now your file looks like this following screenshot
Step 2:
Run following command. It will generate main.jsbundle file
yarn build:iosornpm run build:ios