LR 12 Catalog will not open in Mac studio but does in LR 11.5

LR 12 Catalog will not open in Mac studio but does in LR 11.5. The catalog in question is on an external drive. I've tried to optimize the catalog and tried saving a backup catalog to the internal HD, which is grayed out if I try to reopen the backup in LR 12. The catalog in question is huge so it is not possible to export to my internal HD. A small LR catalog created on the Mac Studio in LR 11.5 DOES open in LR 12. Completely flabbergasted.