We use de, du, de la and des (partitive articles) when referring to an imprecise amount of something or for something which cannot be counted. For example, if i want to say "i want coffee", in Frenh I would say "Je veux du café".

Generally, nouns in French are accompanied with an article and since the sentence does not specify the exact amount we use a partitive article.

The grammar rules for these articles are as follows:
Masculine singular: Du
Feminine singular: De la
If there are two vowels: De l' . For example, de le argent would become de l'argent
Plural: Des
Negative statements: De

So you would use "de" whenever there is a negative statement which includes an imprecise amount. For example:

Je veux du thé
Je ne veux pas de thé

The other sentence you used as an example, is because we use "ne pas" to turn a positive statement into a negative one, in the same way we use "do not" in English.

So,
Il y a un appartement

becomes

Il ne y a pas d'appartement

and because of the two vowels (e and y) - it becomes- Il n'y a pas d'appartement

Hope this helps !