1. Run react-native run android/run-ios.

if you see Build failed,
Run: cd android && ./gradlew clean

Rerun: react-native run android.
Then you will see build successful!.

2. But if you see error like:

"Could not connect to development server".
On your terminal with project file,
Run: react-native start
Boom!!!

3. Any Extra errorS: like "You need to accept sdk manager licenses"

Run : cd C:\Users\HP\AppData\Local\Android\Sdk\tools\bin on command line and;
Run : sdkmanager --licenses
Click (Y/N): Y(yes) to accept all the sdk manager licenses (SKADOOSH)
Re-run: "npx react-native run-android".