Just in case anyone else here runs into this problem:
If you've gotten TS2:UC free from Origin & out of nowhere, you go to play your game & get "The Sims 2 is not installed on this system, or some required files have been deleted. This expansion pack requires The Sims 2 in order to run. Please install The Sims 2 before attempting to run this application.", you should apparently go into Origin > My Games & right-click TS2 & choose "Repair Game." Evidently this makes it all better. I wish I'd checked
http://modthesims.info/t/529315 before I'd uninstalled & reinstalled it the first time I got that error. I can't vouch for the efficacy of the repair, because it's going as I type, but I hope this helps anyone who gets this scary error.
Comments
You'll have to buy all of the games that you had had installed in order to fix it and hope your computer plays nicely with the disc version and that all copies you buy have the code.
EA is no longer giving out UC codes so unless you can get access to your original account, you're going to have to purchase the game again.
EDIT: Unless you're saying that you've got the disc version installed and it's pulling that error. In which case you'll have to uninstall and reinstall and hope it clears the issue up.