Hey Daniella,
If you ever get an answer please let me know 😁
I've seen this error many times before and it usually just means you have to try again or restart the service or use the validate network topo GP tool. I've also just seen it mysteriously go away the next day and start working. It can be extremely frustrating for sure.
What version of Pro are you using as well as Enterprise? I know at 10.8.1 (install the patches 1-3) it has gotten much better. I would suggest the following:
-Close out of Pro and add the UN to a blank map and try again. If a small area use the ribbon validate. If a larger area use the GP tool.
-Start Stop the Feature Service as well as the System| UN service.
-Take a long walk...