@Olivesplum06 It looks like Origin doesn't sell the base game separately anymore, although you can of course buy it somewhere else and then add the code to your account. So instead of requiring you to own a product that it doesn't sell, Origin lists the starter pack, which is the only combo pack available (and the only product that contains the base game, which is of course required).
You're supposed to be able to buy, download, and install other packs without owning the starter pack specifically; you're also supposed to get credit for owning the starter pack if you own its components (Late Night and High-End Loft Stuff). In practice, some players run into the occasional error, but it's usually resolved by either clearing Origin's cache, or manually uninstalling and reinstalling Origin itself.
https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/
https://help.ea.com/en-us/help/origin/origin/manually-uninstalling-origin/
If you still can't buy packs from Origin, please post about it at AHQ, where EA staff are more likely to see your thread and forward it to the appropriate destination.
https://answers.ea.com/t5/The-Sims-3/bd-p/the-sims-3
