Expansion Packs stuck in preparing mode

Please restart your computer, clear the EA App's cache:

https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/

run the App as an admin (right-click the shortcut and select "Run as administrator"), and try again.  If you use a third-party antivirus, disable it while trying to install the packs.  As long as you don't do anything else at the same time, your computer should be safe.

If the packs are still stuck preparing, uninstall Sims 4 and the EA App with Revo Uninstaller (the free version is fine).  Restart your computer, reinstall the App, and create a new folder into which you'll install Sims 4.  The folder should be on the root level of the drive of your choice, for example C:\Games is fine, but not C:\Program Files\Games.

Run the EA App as an admin again, and once again, temporarily disable your antivirus while installing.