I can't get the game to recognize that I built to the world limit. I saved and closed out. But still no luck. I'm in my son's realm: is that the reason it won't work?
The world’s limit must be reached via scaffolding.
I believe this achievement requires reaching the sky limit with scaffolding. Fortunately, you can build all the way up, minus the top block with any regular material (dirt, cobble, etc), and then use scaffolding for the very last block, and the achievement should pop*.
*Some achievements in this game are slightly bugged and sometimes don't pop. I've had a few like this. In my experience, if you're sure you've accomplished the tasks in the achievement, you can either attempt it again, or you may have to start a new world and do it again. Obviously this isn't a great solution, but until the game stops being bugged, there's not much that can be done besides this. Best of luck.