Hi Ensheng,
The new nextgen Operations Dashboard for ArcGIS was not included in the ArcGIS Enterprise - Portal for ArcGIS 10.6 component software install. For the 10.6 release only, Operations Dashboard will have a separate installer for Portal, which should be available next week from the MyEsri site.
So the behavior you've observed is expected until you run the Operations Dashboard installer.
In future releases, Operations Dashboard will be bundled and included within the ArcGIS Enterprise - Portal for ArcGIS component.
Hope this helps,