Randy, not sure if you resolved this already or not, but in my testing of 10.5 pre-release last week I was running into a problem with creating my Portal web addaptor. It ended up that Portal was seeing a web adaptor from a previous attempt or was somehow "corrupt". The folder wasn't in my IIS and the errors I was getting didn't make a lot of sense.
What tech support walked me thru is going into the administration page of Portal so I could delete this phantom install. Once I did that it worked. I'm in the process of uninstalling the pre-release so I can install the final, so I can't look at the exact place, but I had to log in to https://<portalmachine>:7443/arcgis/portaladmin/
Just like ArcGIS Server's admin, caution should be used when using this direct admin site (vs the Manager GUI) since it can also break your install . If you think this might help and don't feel comfortable, you may want to contact tech support to walk you thru it.
This might not help, but just in case, thought I would pass it along.