Kinda late but I'll reply for future learners.
Since 'hiç' means 'none/not at all/not any' your answer should be accepted but the mods have shown that they are not maintaining this course anymore - at least not in the comments.
'Yok' is the opposite of 'var' and 'var' means 'something exists/is there'. Therefore 'yok' means 'something doesn't exist/is not there'.
Neden ışık yok? = Why is there no light?
Neden hiç ışık yok? = Why is there no light at all? (my translation hasn't been accepted either - 22.10.2020)
As a native Turkish speaker it saddens me that the state of this course is demotivating people instead of encouraging them. That way many people will only quit Turkish and not learn it :(