"The girl does not have an orange."
Translation:Kızın bir portakalı yok.
It is working for me... are you using the web-version? Unfortunately, Tips and Notes are not available on the Duo phone app.
Here is the full URL in case that helps. :-)
This gets a little tricky... It's actually: Portakal kızda değil. (literally: The orange is not on/at the girl).
Here is a post by Selcen_Ozturk... Unfortunately, it doesn't include examples of the negative (to not have), but it may help to understand why the structure completely changes when we make the orange specific. If it's still confusing after that, I'll expand on my answer for you :-)
Edit: Just realized I forgot to include the link, sorry about that!