When working with Portal for ArcGIS, Esri basemaps from the ArcGIS Online basemap group does not appear when accessed using Basemap Switcher in Map Viewer. Only custom groups with layers appear in Basemap Switcher. Switching the basemap group from custom to the ArcGIS Online basemap group does not work and displays a spinning loading symbol.
This issue occurs if the Content Configuration setting in the Portal Admin Endpoint is disabled. As stated in ArcGIS Help: Point the portal to local resources, disabling external content automatically disables all content owned by 'esri_[lang]', 'esri_nav', and 'esri'.
To resolve this issue, do the following:
Get help from ArcGIS experts
Download the Esri Support App