Can't bind to 'routerLink' since it isn't a known property of 'a'. ("<header id="header">

This is your app.component.spec.ts

You should also add routerModule there

beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [
        AppComponent
      ],
      imports:[RouterModule.forRoot([])] //add the router module here as well
    });
    TestBed.compileComponents();
  });

When you're testing your AppComponent , like I said , you should give the test bed what ever you've given to your root @NgModule , so :

 beforeEach( () => {
    TestBed.configureTestingModule( {
        imports : [
            BrowserModule,
            FormsModule,
            HttpModule,
            RouterTestingModule.withRoutes( [
                { path : '', redirectTo : '/home', pathMatch : 'full' },
                { path : 'home', component : HomeComponent },
                { path : 'about', component : AboutComponent },
                { path : 'experiments', component : ExperimentsComponent },
                { path : '**', component : HomeComponent }
            ] )
        ],
        declarations : [
            AppComponent,
            AboutComponent,
            HomeComponent,
            ExperimentsComponent,
            ExperimentDetailComponent
        ],

        schemas : [
            CUSTOM_ELEMENTS_SCHEMA
        ],
        providers : [
            ExperimentsService,
            StateService
        ]
    } );
    TestBed.compileComponents();
} );

I had this issue too, and I was using a self-created mock router.

All examples of using/importing the routing, router, RouterTestingModule.withRoutes([]) etc just gave me more errors.

The solution for me was too create a mock for the routerLink directive (I'm not sure where the implementation is found when you don't mock the router but use the RouterTestingModule, probably in the RouterTestingModule itself?)

So in hindsight I could have created the mock myself, but as they say "The real RouterLinkDirective is quite complicated and entangled with other components and directives of the RouterModule. It requires challenging setup to mock and use in tests."

I found the implementation for it at the Angular documentation page here: https://angular.io/guide/testing-components-scenarios#routerlink

As links can go away, I post the code here:

@Directive({
  selector: '[routerLink]'
})
export class RouterLinkDirectiveStub {
  @Input('routerLink') linkParams: any;
  navigatedTo: any = null;

  @HostListener('click')
  onClick() {
    this.navigatedTo = this.linkParams;
  }
}