@PurpleCookie1401 I've moved your post to the Sims 4 general discussion section.
In order to play Sims 4 in Steam, you need to buy the base game in Steam, even if you already own it in Origin. However, any packs you own in Origin will apparently be available when you play the Steam version. So only the base game needs to be purchased twice.
Please keep in mind that if you buy Sims 4 in Steam, both Steam and Origin will run while you play. While Origin does only run in the background, if you have issues with Origin, those issues may not go away just because you're playing the Steam version.