I also have a installation problem using the new ArcGIS Enterprise Builder 10.5.1 on a windows server. Looking in the installation logs all the software successfully installed but during the configuration wizard it fails on 4) Register Data Stores with GIS Server. I get the following error "Unable to register the Portal with the Web Adaptor. Please make sure that that the Portal is running and that the account specified has administrative privileges".
The Portal and server are both running and I can sign into them with my admin account.
Any help would be greatly appreciated.