How do I navigate the ribbon in Office for Mac 2016 with the keyboard?
As of Office 2016, Microsoft has not implemented the Alt-shortcuts for the ribbon.  However, there are several dozen other shortcuts that can be used on the Mac.  

You didn't ask for any specific shortcut so I won't list any here.  Also, there are so many that it would be impracticle to list them out.  I know we try to avoid 'link-only' answers but in this case there's no way around it.  Here are a few very comprehensive lists of all the Office shortcuts for Mac, including where you can use the excellent Accessibility features on the Mac:


Alt key shortcuts on Office for Mac
Keyboard shortcuts in Excel for Mac.  These are very similar shortcuts for all Office apps, too.
Accessibility shortcuts in Excel for Mac
Select menu items, like Alt+letter for Word