the singular or plural should be accepted. also, "however" should also be accepted for "하지만"
The way 없습니다 is used in this sentence is like an adjective so there's no 'object' for it to act on (only verbs act on objects). Instead, when talking about 'possessing' (있다) or 'not possessing' (없다) a thing, you use the ~이/가 markers.
This site explains it very well: https://www.howtostudykorean.com/unit1/unit-1-lessons-1-8/unit-1-lesson-2/ (Jump ahead to the '있다: To have' part of the lesson).