"A coffee without sugar."
Translation:Un café sin azúcar.
It's not a Duo tip - Duo drip-feeds bits of information rather than provided focussed grammar articles.
But there are plenty of freely available resources on the internet.
I use Chrome Web Browser on a Windows PC which has a "Find" tool for searching within the web page (individual sentence discussion)
There is a global forum search function that can be accessed from the main forum page: https://forum.duolingo.com/
I'm pretty sure that other platforms have a "find" capability.
You use una in the first sentence because you are referring to the cup, which has el cafe in it. In th second one, you are referring to el cafe itself. If you are asking why one is feminine and the other is masculine, there is really no logic to it. Nouns that end in o or e are typically masculine, whereas a nouns are usually feminine.