When you complete all tasks from the main storyline on one location, you can move to another land. Once you reach your next stop, the timer sets off, and you have 24 hours to complete everything there. The moment the timer stops, you will no longer have access to that location. This is done in order to optimize the game, so that more players with different devices could play without any issues with RAM or Internet connection.

Don't worry, though! If the game lets you move on to another land, it means all tasks from the main storyline in this location are finished, and you're ready for new adventures!
The key buildings will remain on that land, and you will not be able to access them; however, there are other buildings at your Home Station that serve to make the same items, or there might be an opportunity to make them in the next expedition lands.