It's supposed to be 'δε βρέχει'. Β is plosive, so there should be no νι on the end of δεν.
Did it give you an error? We have a language rule that shouldn't give an error in such cases, so please let me know if this doesn't work.
I'm looking at the translation that Duolingo has, and it says «Όχι, δεν βρέχει.»
I tried "όχι δε βρέχει" and it says "Almost correct, you have a typo in your answer".
Yes, ok that means that the rule works and it doesn't give an error but it gives a typo. In any case, I added both translations now. Thanks for confirming!