it is likely your trust level, which prevents you from sending messages to @Rob and all other people in the forum. this was probably done as a precaution: new forum users should not be overwhelmed by many settings and “experienced” forum users should be able to send messages, because they know what they are talking about.
i have raised your trust level from 0 to 1. normally, you raise your trust level by participating in this forum, i.e. reading posts, writing posts, linking some posts, quoting/answering posts, etc.
just click at the “@Rob” (this or other ones) with your left mouse button and a popup should give you short information about rob. in this small popup, you can click in the upper right corner a “message” button.
please try again to send a PM and tell me, whether you are successful.
if not, i can raise you trust level even further…