You should pay attention to cases of nouns in affirmative and negative statements.
You should use the genitive form of a noun in a negative statement and the accusative form in an affirmative.
Nie mam pieniędzy. (I don't have money) As it is stated above, "pieniędzy"is the plural genitive form of "pieniądz"
Mam pieniądze (I have money) In this case "pieniądz" uses the plural accusative form – "pieniądze"