Translation:The small blue car is moving in between those big buses.
If you have an az or an ez (that, this), then it has to be in the same number and case or take the same postposition as the noun it modifies, as far as I know.
It is difficult to follow the sentence. Is there a possibility to slow down the speed like in the non beta version?
It's not a beta/non-beta thing; it's to do with the fact that most courses use a text-to-speech (TTS) engine (a "computer voice"), but a few (e.g. Esperanto, Irish, Hungarian) have recordings of a real human speaker.
The computer voice can speak individual words as well as full sentences, and if a new sentence is created it can immediately have audio.
But live recordings don't record everything (there's a limit to the number of sentences that Duolingo pays for recording of), new sentences will not have audio, and there is only a single speed.
One would think that the "little blue car" would be an accepted alternate translation. To speed along the transition from Beta to finalized version, I have reported it.
I said "between" instead of "in between" and was marked as incorrect; I have encountered instances where the opposite happened too. I really think it should still be accepted.