It worked for me as well. I use 10.6 version... Thank you for the solution and please #HashTag #OnlineResourcesNotWorking #ArcGISOnlineNotWorking #UnableToAddBaseMap #BaseMapNotWorking
Was using a 10.5 machine and needed a basemap; encountered the same error message. Running PatchFinder indicated I was definitely missing the TLS patch. Installed it and error went away. 👍