Property 'of' does not exist on type 'typeof Observable'.

import { of } from 'rxjs';

import { from } from 'rxjs';

var observableFromPromise =  from(promiseSrc);

import { tap } from 'rxjs/operators';

const TOKEN_HEADER_KEY = 'Authorization';

@Injectable()
export class Interceptor implements HttpInterceptor {

  constructor(private token: TokenStorage, private router: Router) { }

  intercept(req: HttpRequest<any>, next: HttpHandler):
    Observable<HttpSentEvent | HttpHeaderResponse | HttpProgressEvent | HttpResponse<any> | HttpUserEvent<any>> {
    let authReq = req;
    if (this.token.getToken() != null) {
      authReq = req.clone({ headers: req.headers.set(TOKEN_HEADER_KEY, 'Bearer ' + this.token.getToken())});
    }
    return next.handle(authReq).pipe(tap(
        (err: any) => {
          if (err instanceof HttpErrorResponse) {
            console.log(err);
            console.log('req url :: ' + req.url);
            if (err.status === 401) {
              this.router.navigate(['user']);
            }
          }
        }
      ));
  }

}