As always, we won't be able to use the AuthService unless we add the required reference to the AppModule class. Open the app.module.shared.ts file and update it by adding the following highlighted lines:

[...]

import { RouterModule } from '@angular/router';
import { AuthService } from './services/auth.service';
import { AppComponent } from './components/app/app.component';

[...]

imports: [
    CommonModule,
    HttpClientModule,
    FormsModule,
    ReactiveFormsModule,
    RouterModule.forRoot([
        { path: '', redirectTo: 'home', pathMatch: 'full' },
        { path: 'home', component: HomeComponent },
        { path: 'quiz/create', component: QuizEditComponent },
        { path: 'quiz/edit/:id', component: QuizEditComponent },
        { path: 'quiz/:id', component: QuizComponent },
        { path: 'question/create/:id', component: QuestionEditComponent 
                                          },
        { path: 'question/edit/:id', component: QuestionEditComponent 
                                           },
        { path: 'answer/create/:id', component: AnswerEditComponent },
        { path: 'answer/edit/:id', component: AnswerEditComponent },
        { path: 'result/create/:id', component: ResultEditComponent },
        { path: 'result/edit/:id', component: ResultEditComponent },
        { path: 'about', component: AboutComponent },
        { path: 'login', component: LoginComponent },
        { path: '**', component: PageNotFoundComponent }
    ])
],
providers: [
    AuthService
]

[...]

That's it.