Portal for ArcGIS basemaps are not available in ArcGIS Explorer Desktop.
Last Published: February 13, 2017ArcGIS Explorer
Bug ID Number
May 30, 2015
June 11, 2020
After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
Specify a basemap gallery and put Explorer maps in it. http://webhelp.esri.com/arcgisexplorer/2500//en/index.html#