I unzipped arcgis-web-appbuilder-2.21 to my desktop ..\Desktop\arcgis-web-appbuilder-2.21\ArcGISWebAppBuilder and ran the startup.bat. It opened the browser at http://<mymachinename>:3344/webappbuilder/?action=setportalurl.
I entered a url of our portal and a app id. Then clicked the 'APPROVE' button. But it doesn't show the Web AppBuilder to customize.
Thanks a lot.