Translation:Today, I am going to climb a mountain.
Probably just because it sounds a bit weird to say "I climb a mountain today"; in simple present tense, the English sentence sounds more like a plan or statement of intention, which would be translated differently in Japanese. I think Duo should still accept the simple present tense as a translation, though.
I don't think it is wrong. But in this course, Duolingo tends not to recognize that in English, present progressive tense is one very standard way to talk about events in the future, maybe even more often than it's used to talk about events in the present. It's strange that they don't, considering in Japanese the plain present tense is used much the same way. But not present progressive.
I think they're trying to help us get used to certain verb forms by consistently comparing them to something familiar, but when these forms only actually have the same usage in both languages half the time, it can be more confusing than anything to try to map them literally to one another, or even to refer to them by the same name.
I am not english, so i am a bit lost in translation here but i have 2 questions: 1) if duo want to translate it as : i am going, in progressive, why japanese sentence use noborimasu, ¿instead? Of something like te form (¿Nobotteimasu? If nobotte is correct, i don't know); i can understand more or less, if i say -te form then i will say : i am climbing but then, 2) how can i say : going without -te form? Ok, almost sure i am wrong with the -te form, because i think it is the way to say i am «doing something» right now. Can you people understand me and help me, please? Thank you so much.