After I hit the green “Build” button, the screen keeps saying that the world has not been built, and it won’t let me save the world in the file output box. How do I get the world finalized so that I can save it?
Hi, check this other thread here. It might have the answer to your problem… If it doesn’t, let us know 
detailing a bit more, if you can.
It might be you not-selecting a device before hitting the green button. The green button builds everything and then displays the result for the selected device… if none is selected nothing appears…
Cheers,