Translation:Who is able to explain the system to me?
'To explain x to y' is not 'at forklare x til y' but 'at forklare x FOR y'. But even that is bad Danish. 'Hvem er i stand til at forklare mig det?' would be much better. Further: We don't say: 'er i stand til'. It's correct, strictly speaking, but we don't speak like that. The correct, idiomatic translation of 'Who is able to explain the system to me?' is: Hvem kan forklare mig systemet?
Ok, I was about to ask why "Who can explain me the system" is considered wrong, but it seems to me that the sentence just doesn't make sense to start with...
"Hvem kan forklare mig systemet?" Although 'kunne' CAN mean 'to be capable of', it doesn't (really) carry that meaning in this sentence. In stead, this is a 'please'-question, and the meaning is: 'Who would like to (please) explain the system to me?' Danish often uses questions where English would use 'please'.
Another example of the same thing: Idiomatic Danish: 'Kan jeg ikke få dig til at åbne et vindue'? Literally: 'Can't I get you to open a window'? Idiomatic English: 'Please open a window'.
In short: A Dane using 'kunne' like this is not inquiring whether you have a certain capability, rather he is asking for a favor. :)
My translation was "Who are able to explain the system to me" which was marked wrong. But I think that "hvem er" can also have a plural meaning, even if in most contexts the singular seems more appropriate.