Translation:It's not eight o'clock now.
Its formal. Most people will opt for じゃないです (more informal) or even just じゃない (very casual).
Report it as an error, but the reason may be down to using "right now" instead of just "now". Still, I don't see any reason why "right now" shouldn't be accepted.
Can i replace every 「ではありません」with「じゃない」because I think that'll be simpler and more natural.
Instead of 「じゃない」, it might be better to use 「ではない」; I usually thought「じゃない」 is usually (just) used after adjectives (studied adjectives [and nothing else] for WAAAAY TOOO LONG... apparently), but it is just as good I'm sure (but what would I know, 3 years in study and never studied 「です」 long enough to know 「じゃない」is also casual...) Although, I agree with you; 「ではありません」sounds a bit too formal to be natural.
Why would you respond stating that it is not the correct time. If someone says 'its 7 o'clock'. And you reply 'no it is not 7 oclock.' dot dot dot...you win the Nobel prize for most useful response of the year. Sentence could use some better context.
can someone explain what the "de" means? sometimes they just end with "arimasen" without the "dewa" and i dont really understand it
ではありません is the negative of です. So just like です will often correspond to "am/is/are/etc." in English, ではありません would be "am not/is not/are not/etc." So for example, 八時です would be "It's 8:00," while 八時ではありません would be "It isn't 8:00."
ありません by itself (no では) is the negation of あります, which is the word for "to exist/to have" for inanimate objects. So ありません is "doesn't exist/don't have" for inanimate objects. For example, if you went to a restaurant and asked if they had a hamburger, they might respond ありません if they don't sell hamburgers.
I put "it is not eight o'clock right now" and apparently that's wrong...
Sorry this may be dated but if others are looking for a reply to this, the 'は' between '今' and '八' is because 'は' is a subject particle and whatever comes before it is usually the main topic or subject of the sentence. Since '今' comes before 'は' in the sentence, we make it clear to the reader or listener that we are talking about right now (since '今' is equivalent to saying 'now' in english). You can say '八時です' meaning 'It is eight o'clock' (implying it is eight o'clock now) but to make it more assured that we are talking about now, we add '今は' to the beginning of the sentence!
I am very new to the language so this may not be entirely correct, but I hope this helps anyone who is looking to understand more about particles. There is a helpful guide to particles (plus lots more) on youtube from japanesepod101 if you are looking for more information!
What does it means when "you have a typo in your answer"? I'd be glad if you tell me.
I think it is the contraction (i.e. "isn't" instead of "is not") because I answered "it is not eight right now " and it was accepted.
I put "Now it's not eight o'clock" and DL Japanese said I had a 'typo" Where??????????????????????????
"Now it's not eight o'clock" is an unnatural English sentence. Now should be at the end, although that would be superfluous. Perhaps the proper option would be "It's not eight o'clock [right now]" with the  being optional/uncommon/informal. Also, Duolingo shows you your typos at the bottom of the screen.