I'm assuming the German is an idiom of some sort? I assumed the sentence was "You must only ask afterwards", in the same sense of someone giving a presentation and asking the audience to hold all questions until the end. Or am I completely missing a possible transaltion?
But should "You must only ask afterwards" still be right huh? I came up with the same translation but it was marked wrong.
To be honest that doesn't sound like something anyone would ever say. I came up with a similar answer as you because I didn't know the idiom, but the idiom sounds 100x more common.
In the example Honemens gave, the presenter would say something like "I will answer your questions later" or "Hold on to your questions until I'm done" or "There will be a Q&A session at the end".
I just got 'You must only ask afterwards' marked as correct - so they must now be accepting it as a valid translation BUT - that means I would have missed out on it as a colloquial phrase if I hadn't looked at the discussion! I like this site but it drives me crazy sometimes.
Another way to read the sentence is to read "danach" as "da + nach". The structure "nach etwas fragen" is like the English "to ask for something". In this case, since "something" is "it", rather than saying, "Du musst nur nach ihm fragen", you can instead change "ihm" to "da" and combine "nach ihm" as "danach". If this is confusing, a quick internet search for "da compounds" should help to clarify the issue.