No rule for this one
No ending or word group predicts the article of Chef. Learn it as a pair and say it out loud: der Chef.
der Chef
It's der Chef. Chef is masculine, plural die Chefs.
Mein Chef heißt Herr Schulz.
My boss is Mr Schulz.
No ending or word group predicts the article of Chef. Learn it as a pair and say it out loud: der Chef.