Translation:I would like to take a break.
Whilst I wouldn't say your sentence is wrong, I think it's a bit misleading and I see why it was rejected. It's quite easy to mistakenly think "一下" means "a bit/a while/a little" but it doesn't at all. It's just a way of softening the tone of the sentence to sound less demanding and more polite. (Especially when making suggestions to people or asking for favours).
Somtimes 一下 is used to soften the tone, but that is usually when talking to other people. For example, 请看一下 is "please take a (quick) look." The speaker is making a polite request or command of another person.
In this sentence, there is no need to soften the tone when you are speaking about yourself.
On it's own, 休息 means "to rest" or "rest." The use of 一下 shortens an activity. To take "a short rest" equals "a break." That is why Duo translated 休息一下 as "a break."
休息一下 means "rest for a bit", "a short rest", or "a break." In this context, it is not being used to soften a command or request.
I welcome corrections from a native speaker if I have misunderstood.