The build doesn't work as inteneded, I assume it has something to do with how you loaded in the save file. (if ur using json or yaml to save data)
When the player first opens up the game, you tried to get the save file. However, the game doesn't have a save file to access, so it gives off errors and makes the game completely unplayable. You need to create the file if it doesn't exists. The reason why it worked for you is because when you playtested the file already existed on your computer.
It's all just speculation anyway, hoped this helped!