Translation:I am good today.
Hi Jack, I think it's because of the "very" that it's marked incorrect. I don't speak Mandarin nearly well enough to know how fluent speakers speak it, but at least in Duolingo (possibly in Mandarin as well, assuming the app is correct) 很 is a substitute for "is." So "Today I am well" or "I am well today" should both work (I've only tried the latter). But any sentence that adds "very" is marked incorrect.
In English we have several options for word order, but we only know which ones are correct because we've been exposed to them over time. Until you speak Mandarin fluently, stick with the word order that is presented in the app or website. It will make your language learning much easier.
Edit: According to google translate, atleast
Why as far I study Chinese in Duolingo, Feel like "很" doesn't use in English, even through it's have own meaning. The meaning of "很" is "Very". Okey then, if you don't use word "very". So, we will say more better word instead use "very" word. Like the example Cheerful instead Very happy (很高兴)