I had the version of this question in which you choose the correct translations from among three possibilities. 'Quando arrivo là' was one of the correct ones we had to select, so I don't know why it didn't accept it from you. :(
I had 'Quando arrivo li (with an accent)' accepted. Maybe they accept 'la' now as well. I'm unclear as to whether 'ci arrivo' is preferable to 'arrivo la/li' - perhaps someone with a better sense of Italian colloquialisms can comment.