It could be a firewall issue. I understand that ArcGIS Pro initially communicates with the Enterprise Portal using HTTPS (Port 443) which is typically open. However, it then communicates with the license manager using 27000. If your portal machine has a software firewall, then this will need to be opened. If your License Manager is in a different network security zone to the ArcGIS Pro, then you will need to open 27000 on the perimeter firewall as well.
I'm not saying this is the answer, but it's something that is often overlooked.
Scott Tansley
https://www.linkedin.com/in/scotttansley/