Then you don't have any empty slots (visible in your boff list as gray slots below your last commissioned officer). You can dismiss ones you aren't using to replace them, or buy more slots. Boff slots are one of the more restrictive limitations on silvers.
I see...I think. Why would you be able to have a ship with 4 bridge officers if you're not allowed to have 4?
Don't take this wrong...I'm not understanding, really.
Where do you go to get Bridge Officer slots added?