Dynamically creating a component in DOM location

app.component.ts

@Component({
    selector: 'app-root',
    template: ``
})
export class AppComponent implements OnInit {
    private componentRef;

    constructor(
     private componentFactoryResolver: ComponentFactoryResolver, 
     private injector: Injector
    ) { } 

    ngOnInit() {        
        // Locate an element that exists on the page
        const headerElement = document.querySelector('#pageHeader');
        // Locate the component factory for the HeaderComponent
        const componentFactory = this.componentFactoryResolver.resolveComponentFactory(HeaderComponent);
        // Generate an instance of the HeaderComponent 
        this.componentRef = componentFactory.create(this.injector, [], headerElement);
        // Attach to the component to Angular's component tree for dirty checking
        this.applicationRef.attachView(this.componentRef.hostView);
    }
}

You forgot to import ApplicationRef

 constructor(
     private componentFactoryResolver: ComponentFactoryResolver,
     private injector: Injector,
     private applicationRef: ApplicationRef,
    ) { }