Question

Why we Dropped (das) article in the sentence?

ich möchte für nächstes wochenende ein zimmer reservieren​.​

Answer 1

the answer is simple; we dropped (das) and the added (ein) because we were not referring to something special and there's no certainty for it.

for example, in English, we say: I like to reserve a room for next week. ( Note that we are not looking for a special room and we just want a room).