Translation:Is there a police box in this town?
交番 are still everywhere in Japan. They aren't like the police boxes we used to have in England; theirs look more like a small post office with a police officer inside waiting to offer assistance to anyone who needs it, but also can be used as a branch for processing if someone is suspected of criminal activity.
On a related note, more often than not if you get stopped by police they'll just want to practice their English with you and are often quite happy to talk to foreigners. That aside, always, always, always carry your passport or residence card (在留カード), the police can ask to view identification at any time. Also, don't forget to ask them where the best drinking spots are, unless you're under 20. They'll be happy to oblige!
In my opinion, Duolingo has overused は (topic marker) and underused に (location marker, amongst other things) and が (subject marker). In this given sentence, the は is indicating that the こうばん is the topic (the main focus) rather than このまち. Here are 3 examples of saying very similar things (they could all be translated as 'Does this town have a police box?' in English).
このまちはこうばんがありますか = As for this town (as opposed to other towns), does it have a police box? (The topic of conversation is 'this town'. The context may reveal the people have been talking about this town. It may also indicate they are comparing this town to other towns)
このまちにこうばんはありますか = As for police boxes (as opposed to other buildings/services), are there any in this town? (The topic of conversation is 'police boxes'. The context may reveal the people have been talking about police boxes. It may also indicate they are comparing various locations/services in the town) * I would probably put the は phrase first, so こうばんはこのまちにありますか seems more natural to me.*
このまちにこうばんがありますか = Does this town have a police box? (There is no topic of conversation yet. 'Police box' is the subject of the sentence but it is the first time it has been mentioned in this conversation. This is either a change of topic or the start of a conversation, probably somebody asking for directions)