npm start or
npm run webpack:build.
npm install. If you are behind a corporate proxy, it is likely that this error was caused by your proxy. Have a look at the JHipster error logs, you will probably have the cause of the error.
If you want to go fast, run
./gradlew to build and run everything.
If you want to have more control, so you can debug your issue more easily, you should follow the following steps:
npm run webpack:build or
./gradlew or using your IDE