Hi Rachel @rayenic ,
Make sure that the whole Mesh is 1 Object (if not: Switch to Object Mode. Select the 2 or more Objects and press CTRL+J. Then return to Edit Mode and try again.)
If you have Added the inner parts (screen & Co.) in Object Mode, they are their own (separate) Objects. You can be in Edit Mode on more than 1 Object at the same time, but you cannot connect them (with F, for instance).
