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