Sometimes Duolingo rejected the answer when you make a little mistake, mainly when I put the wrong article. Duolingo is often hard
There's an archaic verb śniadać, but apart from literature from before 1900, you won't find it in real life. Using it in speech would be very confusing.
Why is "your duck eats breakfast" wrong? I thought is eating/eats were interchangeable
Your duck eats breakfast is a general description, but "is eating breakfast" is specific to this instance. It's something my Polish girlfriend gets wrong sometimes i.e. I eat breakfast, when she means I am eating breakfast. It's not technically wrong, just sounds clunky.
But this clunky answer is accepted, because we have no further context. "is eating/eats" aren't of course really interchangeable, but they both are (almost) always accepted as translations in this course.