ArcGIS Pro 2.3: Are we going to publish directly to ArcGIS Server in Pro 2.3 without the need to have the Portal installed?
I have not idea if we are we going to publish directly to ArcGIS Server in Pro 2.3 without the need to have the Portal installed. This would be a great enhancement in my eyes!
The answer to this question is in Idea "ArcGIS Pro Publish services to ArcGIS Server". I believe you would need to upgrade your enterprise environment to 10.7 to get this to work.
You should be able to publish to stand-alone server at 10.6; Connect to a GIS server—ArcGIS Pro | ArcGIS Desktop
More information on doing it from ArcGIS Pro 2.4.x; Publish a map service—ArcGIS Pro | ArcGIS Desktop