@norah414 Please repair the game in the EA App: open your game library, click Sims 3, and select Manage > Repair. Please also download a fresh copy of one custom content item to test with.
If that doesn't help, clear the Origin and EA App cache files:
https://bluebellflora.com/how-to-uninstall-origin-on-a-mac/
https://bluebellflora.com/2024/04/04/ea-app-recovery-for-mac-reset-ea-app-by-clearing-cache-and-other-files-manually/
and repair again. Then try to install a new cc item. Please also try installing a Store item; Riverview and some holiday sets are free if you don't own anything. I'd like to know whether you get an error, and if so, which one.