why not j'ai du liquide?
Is it because one means "cash" and the other means "change"? I made the same mistake, by the way.
what is the need to have here "de"?
monnaie → "change" or "currency" (in the sense of coins) is a mass noun so requires a partitive article. As monnaie is feminine, the partitive article is de la.
Why not "J'ai des pièces"? As in "Tu me rends trop de pièces" that also appears in this course?
I think that is the French for "coins".