No rule for this one
No ending or word group predicts the article of Balkon. Learn it as a pair and say it out loud: der Balkon.
der Balkon
It's der Balkon. Balkon is masculine, plural die Balkone.
Die Wohnung hat einen Balkon.
The apartment has a balcony.
No ending or word group predicts the article of Balkon. Learn it as a pair and say it out loud: der Balkon.