So the secure services is hosted on ArcGIS Online, as a hosted service? If you were to run Fiddler or the Dev tools while adding the service, do you see a checkURL request? Take a look at the response and see if it can successfully connect to the service. If not, take a look at the Portal logs to determine if there's a trust issue with certificates, (unlikely if the service is actually hosted on ArcGIS Online), or if there's simply a connection issue. If your Portal doesn't have the ability to make outbound connections, (in this case, to test the accessibility of the service you're trying to add), you may run into issues.