Translation:I have not been here for three years, it has changed a lot.
I think the problem is that the Chinese sentence needed some restructuring to be put into good English, but there are any number of ways to do this. How many versions will be accepted? I chose to try a more literal, less idiomatic English translation this time and was rejected. I should have gone with my gut feeling, which was pretty close to what they suggested.
Technically it would mean something like "I didn't come anymore in three years", but it sounds a bit weird to me. This kind of sentence is often used in the present tense, for example:
我不抽烟 "I don't smoke"，我不抽烟了 "I don't smoke anymore"
However, I'm not sure if you'll often hear 我沒抽烟了 "I didn't smoke anymore". This kind of construction also sounds somewhat weird to me, not sure if it's correct.
Duolingo is rather restrictive about translations for this sentence. It insists on "a lot" as well as "come". In this case the issue might be the "been" part, because 来 translates to "come". However, I think "been" sounds more natural, it's just that the translation is not as close. On the other hand, I have reported sentences with "been" instead of "come".
The 的 turns 这里 into an adjectival modifier of 变化. In standard English we can't use 'here' as an adjective in the usual pre-noun, attributive position (though some dialects allow combinations with this / these, such as 'this here book'), but we can use it in the post-noun, postpositive position: the change(s) here. So in the Chinese version, the word 'here' actually appears overtly only in the second clause, modifying change(s), not in the first, although it is implied there, to give a destination to 'come'. However, I think the content creators here would argue that not to give 'come' an explicit destination in English would seem less natural, making 'here' in the second clause redundant.