The Sietch requires that users make at least 25 posts on the forums before they are able to send PM's. This is an anti-spam measure.
Do you see the little envelope at the top right of the screen, next to your name and to the right of the alert bell?The PM function doesn't seem to be working for me. Can't find any way to send one, either in the PM tab or by clicking a user's profile or from a user's profile page.