I'd like to plot to kill my liege but its not available as an option. How can I make it so?
Have you checked his character page? The actual Intrigue screen will usually only list people you stand to inherit from and your wife, however every character in the game has a personal 'plot' button on their character page to the left of their portrait and right below their realm coat of arms. That button will nearly always have the plot to kill them (will be greyed out if you already have an active plot).