The Choose Best Facilities tools in Portal for ArcGIS 10.5.1 which are enabled after configuring portal to use network utility services from ArcGIS Online do not execute.
This issue is not reproducible and is addressed in Portal for ArcGIS 10.6 or 10.6.1. This workflow is tested and verified to work in Portal for ArcGIS 10.6 and 10.6.1.