Translation:A big car is waiting in front of the stop.
That works too!
Your sentence might put more emphasis on 'egy nagy autó', but not necessarily. The empathic position is always directly in front of the verb and both your sentence and Duo's preferred one have 'egy nagy autó' there. Since in yours it's also in the first position, there might be a bit more emphasis on it, but intonation would have a major role here.
It seems to me "is waiting" versus "waits" couldn't cause problems for this exercise. Both should be accepted. If your solution wasn't, you ought to suspect there was something else in your sentence that isn't accepted. Now that we have come this far, may I ask what your solution actually was?
I only dare to spread my speculations on the topic because noone else seems to even try to explain the difference even though everyone seems to agree there is a difference.
Without "egy", it sounds like you expected something waited there and you identify it as a big car. It would be unnatural to start talking about that car because it hasn't been introduced as a new thing - therefore it would sound quite uncommon and useless, you probably want to elaborate about that car.
In my opinion, it's a bit like "egy" version: "Let me introduce a new thing in my story: it's a big car and the reason to talk about it is that it's waiting in front of a stop"
no "egy" version: "So as you could guess, there is something waiting in front of the stop - I found out what it was, some big car"