This error I got in Angular 11 is actually pretty basic. You are trying to loop over a component, for example:

  <div *ngFor="let book of books">
    <app-book-card [book]="book"></app-book-card>
  </div>

But the component <app-book-card> does not know how to read the variable [book] you send with it. 

Solution

In your component, in my case book-card-component.ts, add an input decorator:

export class BookCardComponent implements OnInit {
  @Input() book: any;

  constructor() { }

  ngOnInit(): void {
  }

}