I see that beneden also translates as downstairs, yet Don't look downstairs is wrong. What makes it wrong?
(I did not report it because I'm not convinced that my translation should be considered correct.)
Downstairs is used in a building, going down the stairs. De trap af, naar beneden (gaan). Beneden is also down a hill, what you see from a tower or a bridge.
The word "beneden" does not really mean "downstairs". It's just that certain Dutch sentences that use it can be translated into English with sentences that use "downstairs".
As you can see by looking at the Dutch word, it is cognate to the English "beneath" and its literal meaning is: down, under, below, beneath, etc.
But, for example, instead of translating the Dutch "De badkamer ligt beneden" into English as "The bathroom lies beneath", it would be more natural to translate "The bathroom is downstairs".
In the Dutch sentence we are given here, we are just saying "don't look down". The sentence has nothing to do with the different levels of a building.
The same: "Niet naar beneden kijken!" You could also say "Kijk niet naar beneden!" which is probably a more accurate translation for "Do not look down!"