Duo accepted "We are resting now"
That is much more idiomatic English. "We now rest" is not the way such an even is expressed in American English.
Part of the problem is that simple present is most often used to express habitual or re-occurring actions or events, or to express general states which are not limited to the current moment. "We rest now" speaks to one event only, happening at this moment, which conflicts with the usual habitual usage.
Also, current action or events are usually expressed using present continuous, not simple present: "We are resting now".
I'm not certain of the rule about adverb placement, but "now" generally should go after the verb in present continuous. If simple present is appropriate, "now" appears before the verb, thus, "Duo now accepts "We are resting now"."
I could state this as a general rule that adverbs appear before simple present verbs and after present continuous verbs, but I'm not sure if that's generally valid. It seems to be.
Standard English usage norms would tend to only use that sentence if it were to give an order to a group (imperative) or as a strong suggestion. It would not be used to describe what is happening at the present time. For that function the preference in standard English would be to use the present continuous/progressive i.e. We´re resting now.
I understand, as I understand sometimes in excercises you find homonyms being accepted as good translation even if in meaning is not the correct one in the final language (for example лук may be inserted in a phrase meaning onion and it may be accepted if I translated as "bow"). I guess this is a similar case: the pupil doesn't know if the guy that created the exercise wants to express this or that meaning: he only cares about giving a valid translation, and I guess this one should be valid.
We now rest is correct English. But it implies once only. We are resting is the format that implies this is something that you have done before, are doing now, and may do tomorrow. They now rest from their labour is a common English phrase; but it is usually understood to mean "they are dead". They are resting now, and will continue to rest eternally.
Native speaker of American English here, and I've never "had a rest" in my life. I've rested, I've taken a break, I've had a chat, but I've never had a rest. I think "we rest now", "now, we rest" are my top two translations..."we now rest" sounds awkward but literal so I'll take that one too. "Having a rest" seems unnecessarily regional and colloquial. Acceptable maybe, but not the primary answer.
It's proper English, but the translation is a little different. "We rest now," sounds like a command, like you wrote Gandalf would say before they rest. Мы сейчас отдыхаем, is not a command. It's a statement of something that's already happening. "We are resting now." Two different meanings.
"We are having a rest" should also be accepted, the "now" isn't necessary as it's already in present continuous.
Weighing in on an age old battle - "We rest now" is grammatically correct, but rather means that in the past you never rested, but at some point you changed your mind and nowadays you rest from time to time, but not necessarily now as made clear by "сейчас".
'We rest now' or 'We now rest' are both acceptable. The translations are generally very literal and, sometimes, unnatural; that's how Duo tends to work. 'We are having a rest now' is more idiomatic in British English and flows well, but the other more stilted offerings ought also to be acceptable. Reporting on 27th May 2020.
It's a very uncommon way to phrase that in English. "We rest" and "We're resting" have a slightly different meaning, and the "now" provides context that it's present continuous, not simple present tense. If it said something like "every fifteen minutes," then the simple present tense would be used.
As a native English speaker (US), none of these sound natural. As mentioned above "we rest now" sounds like something said in a movie by a non-native speaker as a command. "We have rest now" could be changed to be "Now we have rested", but that changes the meaning from the exercise translation. This would indicate that the action of resting was completed. "We are having rest now" has the same problem. It is not a construction used in English. To my ear, all these usages of "rest" sound stiff and archaic/ poetic. In conversational English, it would be much more common to hear someone say, "We are relaxing now." However, the "now" still sounds out of place. It would only make sense in specific contexts, as in a response to the question "Why aren't you working?" "We were. Now, we are relaxing/ resting." Otherwise, as a stand alone sentence, it would be much more common to hear someone simply say, "we are relaxing."
Сейчас comes from the two words сей (this) and час (hour). Had it been omitted, they would simply have said "Мы отдыхаем" but adding сейчас means they're doing it right now, oppose to taking it slow at their country place where they might be doing some work around the car cabin, they are actually sitting, or laying down and resting.
I do agree though that using "rest" as a verb would have been better. Why would Duolingo choose отдыхать = "to have a rest" when it is just simple and more direct to use отдыхать = "to rest."
It's just as senseless to use бегать = "to have a run" or any other Russian verb this way.
That's not a great comparison, though the less wordy and more literal translation would be better. There are plenty of English speakers (including me) who are equally likely to say either "to rest" or "to have a rest", and there are probably people who prefer the latter. I don't know of anyone who would choose to say "to have a run" rather than "to run".