Re: [CURRENT ISSUE 64-bit] Base game incompatible with expansion pack error

@Cxlxnx13  Go into Documents > Electronic Arts > The Sims 3 and delete the file labeled Version.tag.

If you don't see the file, make sure you're in the local version of Documents, not the one in iCloud Drive.  You can access the local version by opening Macintosh HD > Users > your username > Documents.