Unlike in computing’s past, file names can have all kinds of characters in them nowadays. However, some characters still cause problems on certain operating systems. If you’re using a Mac and want to save an Office document, it won’t let you do so if the file name has a forward slash in it.
This is a very, very, very long-running problem that seems to crop up only with Microsoft apps. Create a file in any other program, put a slash in its name, save it, open it, modify it, and save it again. OS X handles it just fine. Do the same with a Word, Excel, or PowerPoint document and you receive an error.
Just rename the file and you’re back in business.