@FindingLainie Restart your computer, then clear Origin's cache and uninstall it "the hard way":
https://help.ea.com/en/help/faq/clear-cache-to-fix-problems-with-your-games/
https://help.ea.com/en/help/origin/origin/manually-uninstalling-origin/
Go into Program Files (x86)\Origin Games and delete the Sims 4 folder there, if it's there at all.
Restart your computer, download the full Origin installer, and run it as an admin: right-click on the download and select "Run as administrator." Here's a direct download link:
http://download.dm.origin.com/origin/live/OriginSetup.exe
Run Origin itself as an admin, and try to download the Sims 4 base game. Please make sure you have at least 40 GB free on the drive where you're installing the game; you can see free drive space by opening a File Explorer window and clicking This PC in the left panel.
