Conditional Formatting on the Mac does not work anymore

Hello,

a rather strange thing: Conditional Formatting on my Excel for Mac 2016 does not work anymore. This is one of my most used functions.
When I click on it, nothing happens. And when I go to Format and click there, a Window drops down but it is empty.
I removed the Excel and installed it again, but the same thing: I can not open it.
I am using Mojave OS.

I used that function ever since I installed Excel and I did not change the OS or anything else.

Any advice on how to solve this problem would be very appreciated.

Thank you.

Dorian

New Info: I was able to solve the problem by updating Excel to the latest version.
Thank you!

Solved. Posting to remove from Unanswered search.