Is it a Word document (*.docx) that you're trying to protect? If it is, and that option doesn't show up, then you probably need to reinstall Office 2016 as the previous reply advised. Since Office 2016 is only available as a download from Microsoft, you might want to download it again using the 2016 Office key you purchased from the Apple Store. Reinstalling Office will not overwrite or remove your documents or templates.