In case somebody also runs into this, I had the same issue (although I created the NWListener differently), and it turned out that the error was caused by an empty listener.newConnectionHandler. Set it before calling start, and it might fix it. Just in case I also had serviceRegistrationUpdateHandler set, although I don't know if it's required.

I would like to point out that there's currently a bug in iOS/VisionOS simulator which causes requiredLocalEndpoint to be ignored! https://openradar.appspot.com/FB13686829 The system simply chooses a random open port instead of using the one provided. Beware! It works fine on divice and on macOS itself though.