Dont know if this would work for you but you can give it a try.
As you are having everything on a single machine i suggest you to
First stop all arcgis services like (Portal Service, Arc Server Hosting Service, Datastore Service etc), Also change their startup type to Manual.
Ensure each service has proper credentials in the 'Log On' tab, the same which was used while installing. It would be safe to reenter the same credentials again for each service.
Check if all ArcGIS folders are having Full access to the user account which was used to setup the Arcgis Enterprise
Restart your PC and than start each service one by one in below order....
wait for atleast 10mins after starting one service and than only start the next one
1) Datastore
2) Arc Server
3) Portal
Try to access the Arc Server Manager via web adaptor and see if you are able to access it now.