I'm in the process of configuring Web Adaptor with Portal for ArcGIS . here's what happened . I Opened the Web Adaptor webpage then I was ordered to select a product to configure it with the Adaptor . I selected 'Portal for ArcGIS' , pressed next , then appeared a webpage with two lines :
HTTP Error 404. The requested resource is not found.
This is exactly what I have done . The authorization process was successful and Portal for ArcGIS Windows service is running fine .