Conditional formatting button not working

Hi - My Conditional Formatting button stopped working (MacOS Mojave 10.14.2, Excel 2016 for Mac, V15.25). This is true for existing files with the formatting done previously and new files. None of them are in shared mode, and all are on my local drive (both hard drive and thumb drive). I made sure no groups of cells were selected on other tabs. Whenever I click the Conditional Formatting button, the only thing that happens is the three color window file control buttons (red, orange, and green at top left of screen) go gray. I saved to Excel 97-04, same issue.
Any help appreciated.

I have to update this question/issue. The problem is actually affecting all ribbon commands. None of the commands, when clicked, give a drop down menu and the red/orange/green button go gray when a function is selected. This severely limits the app's use.
Since this (by title) is significantly different than OP, I am going to open a new thread.
Any suggestions appreciated.