You’ve likely already encountered “beaucoup de” or “peu de” used without an article many times. “Assez de” is similar, but unlike the former I don’t think it can ever be used with an article.

“De” is a preposition in these constructs and the partitive (du, de la, and even the indefinite plural des) is always omitted after it. You can think about other cases like:

Il a besoin d’eau.

Il parle de moments difficiles.

Some other examples:

recueillir un maximum d‘informations

une poignée de données non confidentielles

envoyer un grand nombre de photos

As you can see, words that denote a quantity, such as "assez / suffisamment / beaucoup / maximum / poignée", to name but a few, have a noun immediately follow "de" without a definite article in between.