Don’t get me started on what happened to Sierra! 🙂 In short, yes, Sierra is gone.
As for the issue – what’s the error? I can’t help but feel if you bought it used, that the previous owner may have registered the serial # and thus, it may be tied up under their name? I have never played Nexus the Jupiter Incident, so I am not sure if it checks online against the serial #.
EDIT – Glad you got it to work.
If you ever try some of the older Sierra classic games, I’d recommend: