Angular 6 Error "NullInjectorError: No provider for Router!"

If you are using

RouterModule.forChild(routes)

try it to replace with

RouterModule.forRoot(routes)

First import RouteModule in app.module.ts

import { RouterModule, Routes } from '@angular/router';

And use it like below: (You only import RouterModule but need to add forRoot([]) also.)

imports: [
    RouterModule.forRoot([])
    // other imports here
  ]

If you have any routes use them like below. This sample code from Angular DOC

const appRoutes: Routes = [
  { path: 'crisis-center', component: CrisisListComponent },
  { path: 'hero/:id',      component: HeroDetailComponent },
  {
    path: 'heroes',
    component: HeroListComponent,
    data: { title: 'Heroes List' }
  },
  { path: '',
    redirectTo: '/heroes',
    pathMatch: 'full'
  },
  { path: '**', component: PageNotFoundComponent }
];

@NgModule({
  imports: [
    RouterModule.forRoot(
      appRoutes,
      { enableTracing: true } // <-- debugging purposes only
    )
    // other imports here
  ],
  ...
})
export class AppModule { }

And in your main Component (app.component.html) add <router-outlet></router-outlet>

Hope this will help you!