Translation:In spring, many flowers bloom in the yard.
It might want a more literal translation, since it starts with "はるには" (haru niwa - 'in spring'). には (niwa) is making Spring (はる haru) the topic of the sentence, thus the he translation starts out "In Spring, ..." .
If the sentence was にわにはたくさんの花がはるにさきます。That would translate to putting the emphasis on the yard location or "In the yard, many flowers bloom in spring."
There is a song named 「谷の底で咲く花は」 (flowers that bloom at the bottom of the valley) and 「置かれた場所で咲く」(~ the place where they bloom) which i mostly founded. I know it is not 庭, but why in these cases is it で and for 庭 it is に. In all cases this is the location where the flowers are blooming? I can understand that に is the common usage, but why (for 咲く) in some cases the location where flowers are blooming is given by で and in others it is に, i would like to clearly understand it.
Remark: I did not say that duolingo is wrong here, it is just a question.
Effectively for 庭 itself, i did not find any. That is why it made me a little bit confused.
Sorry for the confusion in my first post, i only talked about 庭, because i did not imagine it could be a specific case.
But, it comes another question to my mind: may it be for some poetic "attitude" that sometimes で is used and sometimes に is used? (I do not talk about 庭 for which, i will certainly not forget now, that it is に...)