You started the application from an IDE and you didn't run npm start or npm run webpack:build.
You had a network error while running 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.
You installed a Node.js version that doesn't work with JHipster: please use an LTS (long-term support) version, as it's the only version we support.
Building the client side code again
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:
Install npm dependencies with the command npm install
Build the client with the command npm run webpack:build or npm start