error NG8001: 'router-outlet' is not a known element

Question

error NG8001: 'router-outlet' is not a known element: 1. If 'router-outlet' is an Angular component, then verify that it is part of this module. 2. If 'router-outlet' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.

4   <router-outlet></router-outlet>

In my app.component the line

<router-outlet></router-outlet>

In my package.json

{
  "name": "auth-table2",
  "version": "0.0.0",
  "scripts": {
    "ng": "ng",
    "start": "ng serve",
    "build": "ng build",
    "test": "ng test",
    "lint": "ng lint",
    "e2e": "ng e2e"
  },
  "private": true,
  "dependencies": {
    "@angular/animations": "~9.0.1",
    "@angular/common": "~9.0.1",
    "@angular/compiler": "~9.0.1",
    "@angular/core": "~9.0.1",
    "@angular/forms": "~9.0.1",
    "@angular/platform-browser": "~9.0.1",
    "@angular/platform-browser-dynamic": "~9.0.1",
    "@angular/router": "~9.0.1",
    "@ng-bootstrap/ng-bootstrap": "^6.0.0",
    "@ngrx/store": "^8.6.0",
    "angularfire2": "^5.4.2",
    "bootstrap": "^4.4.1",
    "firebase": "^7.9.1",
    "rxjs": "~6.5.4",
    "rxjs-compat": "^6.5.4",
    "tslib": "^1.10.0",
    "zone.js": "~0.10.2"
  },
  "devDependencies": {
    "@angular-devkit/build-angular": "~0.900.2",
    "@angular/cli": "~9.0.2",
    "@angular/compiler-cli": "~9.0.1",
    "@angular/language-service": "~9.0.1",
    "@types/node": "^12.11.1",
    "@types/jasmine": "~3.5.0",
    "@types/jasminewd2": "~2.0.3",
    "codelyzer": "^5.1.2",
    "jasmine-core": "~3.5.0",
    "jasmine-spec-reporter": "~4.2.1",
    "karma": "~4.3.0",
    "karma-chrome-launcher": "~3.1.0",
    "karma-coverage-istanbul-reporter": "~2.1.0",
    "karma-jasmine": "~2.0.1",
    "karma-jasmine-html-reporter": "^1.4.2",
    "protractor": "~5.4.3",
    "ts-node": "~8.3.0",
    "tslint": "~5.18.0",
    "typescript": "~3.7.5"
  }
}

In app.module.ts

import { AdminAuthGuard } from './admin-auth-guard.service';
import { UserService } from './user.service';
import { AuthGuard } from './auth-guard.service';
import { AuthService } from './auth.service';
import { environment } from './../environments/environment';
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { AngularFireModule } from 'angularfire2';
import { AngularFireDatabaseModule } from 'angularfire2/database'; 
import { AngularFireAuthModule } from 'angularfire2/auth'; 
import { RouterModule } from '@angular/router';
import { RouterTestingModule } from '@angular/router/testing';

import { NgbModule } from '@ng-bootstrap/ng-bootstrap'; 

import { AppComponent } from './app.component';
import { BsNavbarComponent } from './bs-navbar/bs-navbar.component';
import { HomeComponent } from './home/home.component';
import { ProductsComponent } from './products/products.component';
import { ShoppingCartComponent } from './shopping-cart/shopping-cart.component';
import { CheckOutComponent } from './check-out/check-out.component';
import { OrderSuccessComponent } from './order-success/order-success.component';
import { MyOrdersComponent } from './my-orders/my-orders.component';
import { AdminProductsComponent } from './admin/admin-products/admin-products.component';
import { AdminOrdersComponent } from './admin/admin-orders/admin-orders.component';
import { LoginComponent } from './login/login.component';

@NgModule({
  declarations: [
    AppComponent,
    BsNavbarComponent,
    HomeComponent,
    ProductsComponent,
    ShoppingCartComponent,
    CheckOutComponent,
    OrderSuccessComponent,
    MyOrdersComponent,
    AdminProductsComponent,
    AdminOrdersComponent,
    LoginComponent
  ],
  imports: [
    BrowserModule,
    AngularFireModule.initializeApp(environment.firebase),
    AngularFireDatabaseModule,
    AngularFireAuthModule,
    // NgbModule.forRoot(),
    RouterTestingModule,
    RouterModule.forRoot([
      { path: '', component: HomeComponent },
      { path: 'products', component: ProductsComponent },
      { path: 'shopping-cart', component: ShoppingCartComponent },
      { path: 'login', component: LoginComponent },

      { path: 'check-out', component: CheckOutComponent, canActivate: [AuthGuard] },
      { path: 'order-success', component: OrderSuccessComponent, canActivate: [AuthGuard] },
      { path: 'my/orders', component: MyOrdersComponent, canActivate: [AuthGuard] },

      { 
        path: 'admin/products', 
        component: AdminProductsComponent, 
        canActivate: [AuthGuard, AdminAuthGuard] 
      },
      { 
        path: 'admin/orders', 
        component: AdminOrdersComponent, 
        canActivate: [AuthGuard, AdminAuthGuard] 
      }
    ])    
  ],
  providers: [
    AuthService,
    AuthGuard,
    AdminAuthGuard,
    UserService
  ],
  exports: [
    [ RouterModule ]
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

Answer 1

Here you are using RouterTestingModule this is only use for unit try to use RouterModule

Like this

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

@NgModule({
  imports: [
    RouterModule.forRoot(appRoutes)
  ],
  ...
})

Answer 2

I had the same problem because I was trying to import a Module which would declare a specific set of components. One of those components had a <router-outlet> tag. This was the error during the compilation:

ERROR in src/app/components/layout/layout.component.html:8:17 - error NG8001: 'router-oulet' is not a known element:
    1. If 'router-oulet' is an Angular component, then verify that 
it is part of this module.
    2. If 'router-oulet' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message.

    8                 <router-oulet></router-oulet>

What I did was to import RouterModule on the external module, like this:

@NgModule({
    declarations:[
        AlertsComponent,
        FooterComponent,
        LogoutComponent,
        MessageComponent,
        SearchComponent,
        SidebarComponent,
        TopbarComponent,
        UserInfoComponent,
        LayoutComponent,
    ],
    imports:[
        RouterModule
    ]
})
export class LayoutModule {}

then I imported the module on the app.module.ts

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    AppRoutingModule,
    BrowserModule,
    LayoutModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

Now it's working