I have been able to send PM's in the past but am unable to now. It keeps coming up with "you may not start a conversation with the following recipients" I am trying to answer a question that someone asked me on a thread but am unable to by PM.
It could very well be like CharliRenee said about some users have their settings so that they don't receive messages. If that's the case and you still want to try and message the person about it, then I would just simply ask in the thread if you can do so, or if they would pm you.
You can try sending me a new private message, call it "TEST". If it goes through then then above is most likely. If it won't let you message me, let us know here so someone can get it looked into for you.