Translation:The ground is flat.
-은/-는: marks the topic of the sentence, that is, a thing which we already know about, and about which we are now giving new information
-이/-가: marks a non-topic subject
-을/-를: marks a non-topic object
(When I write -A/-B, I mean that A is the form after a vowel, B is the form after a consonant)
You can also view it this way: Subjects and objects have their respective markers by default, but if they become the topic of the sentence, the topic marker will "eat" that original marker.
What should be the topic pretty much always depends on the larger context, which unfortunately we do not have in Duolingo. This means that Duolingo usually accepts multiple solutions where different things are the topic (or at least should accept; sometimes they forgot to add more obscure options). In this case, 땅은 평평합니다 and 땅이 평평합니다 should be accepted.
Be aware though, that in a real situation where you do have a context, that context will usually dictate ehat should be the topic. In other words, if you say this sentence in context, depending on that context only one of 땅은 평평합니다 or 땅이 평평합니다 would be appropriate. In the first case, marking 땅 as the topic means that the ground is the central thing that you're talking about. In the second case, the groumd being flat is only part of a larger description (maybe you're talking about a landscape and the shape of the ground is just part of a larger picture).