To generate the APK go to the root of the project in the terminal and run the below command:
react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res
Go to android directory:
cd android
Now in this android folder, run this command
./gradlew assembleDebug
You should find the apk file in the following:
yourProject/android/app/build/outputs/apk/debug/app-debug.apk
The thing is, this will generate a "debug apk" to generate a "Release APK" you will have to setup a signature for the APK, but this is only necessary when publishing to the stores, not to create a debug apk version.

Please follow those steps.
Bundle your js:
if you have index.android.js in project root then run
react-native bundle --dev false --platform android --entry-file index.android.js --bundle-output ./android/app/build/intermediates/assets/debug/index.android.bundle --assets-dest ./android/app/build/intermediates/res/merged/debug
if you have index.js in project root then run
react-native bundle --platform android --dev false --entry-file index.js --bundle-output android/app/src/main/assets/index.android.bundle --assets-dest android/app/src/main/res
Create debug apk:
cd android/ ./gradlew assembleDebug
Then You can find your apk here:
cd app/build/outputs/apk/