Several desktop users have reported this error:
After updating ArcMap from 10.5.1 to 10.6.1 they're getting this ArcMap popup error when they try to add data from Portal.
The request was aborted: Could not create SSL/TLS secure channel.
Which SSL and TLS versions are you supporting at the web tier? On the desktop machines, which versions of .Net framework are installed?
My hunch is that as a client .net on your desktop machines wants to connect using TLS 1.2 but doesn't have the right .net cersion to do that. Try installing .Net framework 4.6 on a test machine and see if that helps.
Just verified with esri support:
Fixed at Portal 10.6.1
I'm very familiar with that bug, I helped test the fix that was installed for 10.6.1 - I also helped write the TLS Whitepaper you posted (thanks for promoting it!)
The add data>from portal issue when the server only supports TLS 1.2 should be fixed in 10.6.1 - we're working on patches for previous versions.
Can you confirm your Desktop versions and the TLS versions you're supporting on your portal? ArcGIS Desktop 10.6.1 connecting to a portal that only supports TLS 1.2 should work (my personal portal only supports TLS 1.2, and I run Windows 10 as a client). Seems there's more to this than that bug.
The ArcMap 10.6.1 users that reported this also say they can add data from portal 10.5 from existing mxd files they had before updating to ArcMap 10.6.1
If they try with a new mxd they get that error
Desktop user are running Windows 10 Pro 1709