Technically «дверь в ко́мнату» (with Accusative case) is the door leading to the room, «две́рь в ко́мнате» (with Prepositional case) is the door inside the room, but in most cases these phrases are pretty much interchangeable.
«В» is one of the prepositions that changes meaning depending on whether it’s used with the accusative case or with the prepositional case.
However, since the English translation says 'to the room' and not 'in the room', this course probably won’t accept «две́рь в ко́мнате».
'A door in the room' requires the prepositional case. Дверь в комнате.
'A door to the room' requires the accusative. Дверь в комнату.