Why is there no ArcGIS Pro license to assign the GIS Professional user type in ArcGIS Online?

Answer

The GIS Professional user type comes with access to ArcGIS Pro, ArcGIS Online and a few Apps Bundles. As a GIS Professional user type, ArcGIS Pro licenses are automatically assigned and it is not required to assign the license in ArcGIS Online.

To use ArcGIS Pro as a GIS Professional user type, log in with ArcGIS Online credentials.