Re: Sims 3 64-bit: problems installing store/custom content

@PoppyFoxy  I didn't see your update before posting.  After running the Origin reset tool, you need to repair the game to restore a couple of hidden library files that the reset tool deletes.  When those files aren't present, you get the "critical system failure" error.  To repair, open your Origin game library, right-click on the Sims 3 icon, and select Repair.

Please see if you can download your missing expansion packs as well.  The issue should be resolved now.