As long as you weren't given the audio (des chats and des chattes don't sound alike), it should be accepted as the English sentence doesn't differentiate between male and female cats. If all your cats were female, "des chattes" would be correct. Report it next time as it probably just hasn't been programmed in as an acceptable answer.
There is an audible difference between "de" and "des", but not between "chat" and "chats". If you can't hear the difference between the two on Duolingo, try these for practice and eventually you might get the Duolingo under control. For what it's worth, I can hear the difference (grew up exposed to French though) but I have similar issues with German Duolingo. Sometimes they say three words and I magically hear 1 word that doesn't fit the sentence but I type it anyway assuming that it is some sort of expression in Germany.
Additionally, one wouldn't say "Nous avons de chat". That would sort of translate to "We have of cat".
Hope that helps!
Each one means something a little different.
- Nous avons un chat = we have a cat
- Nous avons des chats = we have cats
- Nous avons les chats = we have the cats
"Du" (m) and "de la" (f) are partitive articles and are used when referring to an undetermined amount of something. In other contexts, these same words may represent "of the" or "from the", but as partitives they are usually not translated to English. Note that, in this sense, the word "some" fits grammatically but it is almost always omitted from the English.
- Je bois du lait = I drink milk. Not "the milk", but an undetermined amount of it.
- Je mange de la viande = I eat meat (an undetermined amount)
- Je mange du pain = I am eating bread
I can only say that when you ask why "des" was added" that you are translating one word at a time from English and you want to translate every single English word into a French word. It doesn't work that way. "Des chats" is how you say "cats" without saying "the cats", i.e., "des" is the plural of "un" or "une" but there is no corresponding word in English for that although some people put "some", it is almost always omitted from the English.