"When do you go to work?"
Translation:Когда ты ходишь на работу?
You say на работе rather than в работе when you're at work, therefore you also use на работу for going to work.
The Tips & Notes for the "Where is it" skill have some tips on when to use на versus в, to some extent it just has to be remembered. https://www.duolingo.com/skill/ru/Where-is-it%3F/tips-and-notes