If you just bought something from and you are trying to download it and its not letting you 
heres what to do.
1.When you go to click on Download, and then a small box pops up saying Error don't worry. Just
  clear you downloads page if you haven't yet. Don't worry you can retrieve them from the Store,
  My Store Account, and clicking on Purchase history.
2.If it pops up with Error again, switch out your game to the actually Sims 3 bases game, your 
  expansion pack may no be capable with the Store for some odd reason.
3.If none of this has worked, then try turning your computer on/off.
4.If none of the above has worked, then re-install your Sims 3 base game.