You must install the following command first
npm install --save @ng-bootstrap/ng-bootstrap
See
https://ng-bootstrap.imtqy.com/#/getting-started
Follow this link, it has a link to the application module and system configuration and what it did wrong
.module.ts
import {NgbModule} from '@ng-bootstrap/ng-bootstrap';
and
imports: [ NgbModule]
systemjs.config.js
**var map:** '@ng-bootstrap': 'node_modules/@ng-bootstrap'
**var packages:** '@ng-bootstrap/ng-bootstrap': { main: 'index.js', defaultExtension: 'js' }