In this Angular test setup, what is the main problem preventing the component from using a mocked DataService?

In this Angular test setup, what is the main problem preventing the component from using a mocked DataService?

beforeEach(() => {
  TestBed.configureTestingModule({
    providers: [DataService]
  });
  fixture = TestBed.createComponent(MyComponent);
  component = fixture.componentInstance;
});