"We do not have any cookies."
Translation:Nie mamy żadnych ciasteczek.
This would work if the word was masculine and the nominative for 'one cookie' was "jeden ciasteczek" ;) But it's neuter, and well, this is how it declines. Maybe someone else can explain why.
On a side note: I don't get why the course authors chose "ciasteczko" as a translation for "cookie", I would simply use "ciastko" as it's the main form, ciasteczko is a diminutive - little cookie...
I think because ciastko is more ambiguous - could be also cake,
but then cake is also tort and ciasto,
and then ciasto is also pie and dough.
Well okay, sometimes ciastko can be used for 'kawałek ciasta', but I'd use it here anyway, as 'cookie' is definitely its first meaning.