Inexact title. See the list below. We don't have an article named Music/ThreepennyOpera, exactly. We do have:
If you meant one of those, just click and go. If you want to start a Music/ThreepennyOpera page, just click the edit button above. Be careful, though, the only things that go in the Main namespace are tropes and should be created through the YKTTW system. Don't put in redirects for shows, books, etc.. Use the right namespace for those.