well, both "без лимону" and "без лимона" are widely used by the native speakers, so in this sense both are correct.
I'm not this course contributor, but I checked the answers anyway - "без лимона" is also accepted as a valid answer. So I don't think there's anything to correct here. Maybe make "без лимона" the best answer to match the prescriptive rule which is hardly being followed by most of the speakers anyway. But "без лимону" should definitely be accepted too, it's not incorrect.