"Read this when you have time."
Translation:Прочитай это, когда у тебя будет время.
First of all, "читай это, когда у тебя будет время", you forgot to translate "have".
No, it is not wrong. But your choice is different.
The choice from the task has a meaning "to read something from the beginning to the end" (because of the perfective "прочитай").
Your choice has a meaning (some kind of) "to read for a while". Your choice implies that the book is realy huge, that it has a lot of information, so your interlocutor has to spend a lot of time to read it. Or, another meaning is that the conversation is about regular mass media, for example "here is a newspaper, it is published once a week, I want you to read this".
Почему нельзя:"Читай это, когда у тебя есть время"??? http://images.vfl.ru/ii/1519757509/cb115f7d/20758969.jpg
Russian is the worst language to learn in Duo, wrong or missing hints, no tips, comments from native speakers not so clear or basically saying "this sentence are different between English and Russian" if are different why use it? I'm learning 6/7 languages on Duo and Russian is the worst for distance, poor classes
It's a mistake to use will after when in English.
See the example "Could you tell me when you know the results"
When we see the phrase, "when you have time", it can be inferred that it refers to a time not in the present but in the future. Even though it is "future time", in English we typically use present tense to express it.
It seems that this is not possible in Russian; instead we must use future tense to talk about future time.