’个‘ is fine for towns, cities, or countries, but ‘座’ sounds better. There are lots of cases that you can use ’个‘, but it is not always safe. For example, you cannot use ’个‘ to count televisions, trees, beds, etc.
I just reported it as well. It's all very well to accept 一個大城市 as a fallback but you should also accept 一座大城市 since many people are learning elsewhere concurrently with Duolingo and might be aware of a more specific classifier.