Can't pull a clip onto the end of my timeline ... (1st time on iMovie) One of the clips has an orange highlight on it. Is this causing the problem?

Question

Can't pull a clip onto the end of my timeline ... (1st time on iMovie) One of the clips has an orange highlight on it. Is this causing the problem?

Answer 1

Ahhh, now I see the problem, Grasshopper.  Problem is, we are all Grasshoppers, here.
Yes, that little piece of info is critical. I'd say the biggest difficulty with FCPX is the whole new language.
If I read you correctly, your Audio is in the Main Storyline (Track 1 in plain old Editorese). This makes anything else you add (above or below) a "connected clip", which we can no longer think of as "Track 2" because Jobs sez so. So, yes, you cannot just add a transition to either end of a connected clip.
No argument with Tracy's solution, because I haven't seen any other general solution presented for any attempt to treat connected clips as the main storyline.  And, of course, when they were "track 1" and "track 2", you could.
An alternative solution, however, is to highlight the connected clip and press Ctrl-V to bring up the Video Animation Editor.  Click the "reveal button" for Compositing: Opacity (bottom, right).  Once open, there is a "fade handle" to the right top.  Drag it left for however much fade you wish.  (there is one on the left as well, but you were only talking about a fade-out.)
A second alternative, perhaps for future projects where everything keys on audio, may be to create a Gap clip (used to be called slug) in the primary storyline before ever dropping in the audio.  Then you can add video to the main story line and modify it without having to go thru hoops to do the simplest thing.