Translation:The lawyer is stepping down from the pavement to the street.
"Onto the street" makes sense too, and it is more literal. But, it is not accepted as a correct translation. Is there a good reason for this?
Strictly speaking "the street" would be "the pavement/sidewalk" and you would step (down) from the pavement onto the road. But I think we can guess what it is they are trying to say here.
If I wasn't trying to use Duo-English, I'd say "he stepped off the pavement into the road"