The Sims 4 Build/Buy Mode Doesn't Work

Try moving your saves folder in documents\electronicarts\thesims4 to the desktop and delete your cache files: http://sims4.crinrict.com/en/2014/09/delete-cache-files/

Your saves are gone when you start the game but you can start a new game again that should work. 

You can then try to start putting saves back to see if it's a particular one that is borking or all of them. Don't put the scratch folder back. You can delete that.