They both work.
As for the reasons, there are two:
- the sentence is already a bit longer than most people would like
- it may be hard to believe how backwards Duolingo was in the day when these sentences were written (~the summer of 2014) , but it did not allow words with hyphens inside.
With в/на, you use the Accusative for direction and the Prepositional for location:
- Иди в парк. = Go to the park.
- Я в парке. = I am in/at the park.
- Я живу в Туле. = I live in Tula.
- Я еду в Тулу. = I am going to Tula. (by vehicle, which is rather natural)
It is arigatō gozaimasu.