How do I enable Boolean modifier?
You have to select an object that is a mesh. Based off your choices I'd say you have a curve or text object selected.
Yes, always keep in mind Blender is selection sensitive. Meaning all the stuff in the menus change based on what object and which type of object you have selected. New menu's appear, the UI changes, new options pop up if you have an armature, a mesh object, if you're in object mode or edit mode. Blender is tricky like that. You get used to it.