I have several perfectly nice arcgis services published and readily visible in rest. Can I add them to my content in ArcGIS Online? Yes! No problem! Can I add them the same way in Portal? NO!
Service " does not exist or is inaccessible.
No it isn't!! It's the same address!!!
C'mon Portal... please?
Strange behavior but I too experienced similar issue.
I managed to handle it using following workaround, copied URL of some already added Service, pasted that URL and replaced Service Name....It worked perfectly fine for me ...I still needs to explore reason behind it.