Can't Register The Sims 3 purchased using Origin on The Sims 3 website

@Tag365Plus  Since you've already used the product key, so to speak, you can't register it a second time on the Sims 3 site.  That registration process is meant for adding product keys from discs or other third-party sellers like Steam, where the game wouldn't already be in your Origin account.

If you login to the Sims 3 site using the same email address and password as you use in your Origin account, does the game show as owned?  If you click My Page, you should see the base game and any expansion packs you own colored in under My Games.  If they're not listed, please clear your browser cache, open a private window, or use a different browser.

Otherwise, you'll need to contact EA customer support for help.  It should be simple enough for a support rep to look up your account details, but you can also take a screenshot of the page where your games should be listed and upload it when you request help.  Support is on limited hours right now, with phone calls entirely unavailable, and live chat only open from 9 am to midnight GMT, Monday through Saturday.

https://help.ea.com/en/contact-us/

Hello. I purchased the game using Origin, so it would not make sense for the product key to already be used. Also, none of the games show up as owned on The Sims 3 website despite me using the same email address as the one used for Origin, and the game actually being considered registered beforehand.

@Tag365Plus  The product key is used in the sense that it's already tied to your Origin account.  You couldn't, for example, take that key and register it to a different account on the Sims 3 website, because part of the function of registering it on that site is to tie it to the underlying Origin account.  (The site and Origin share information, so a key registered in one place is read as already used by the other.)

The point is, since your content isn't showing as owned on the website, you'll need to contact EA customer support to get it sorted out.