After the preposition en there is never (ever!) an article in French. It cannot be so difficult to understand, because with the example it will be clear, we do not use an article in English in a similar context.
This is one of the opportunities where you can try out learning with complete structures. Listen to the sentence and connect this complete structure with the knowledge 'after the preposition en there is no article'. We are quite sure you will not easily forget this after the fifth time.