Except we learned about the construction regarding the verb υπάρχουν. With translation exercises even though there can be more than one possible translation, the most obvious one is preferred. The most obvious one for the one you give is Υπάρχουν πέντε μπισκότα στο πιάτο. True, the sentences are semantically quite similar, but as you get used to DL options, you may appreciate better that sometimes the options are deliberately limited for a variety of reasons that may not be obvious at first.
But you have changed the whole syntax of the sentence. The Greek says: "The plate has 8 cookies." What you have done is interpret the meaning to. A translation is not the same as an interpretation. What you need to do on duo is show the correct version in target language.
Read the comment by D_.. above yours.
Here is a link to the Greek Discussion Forum where you will find a lot of advice.
Look at these:
I can't find a word to directly translate "έχει" that is correct or idiomatic usage, which means that the sentence has to be rephrased into a different syntactic form to be idiomatic English. "There are 8 cookies on the plate" seems to me to be the most obvious way of putting it, altho we could get closer to the original by writing "The plate has 8 cookies on it."