Sigh, There are three solutions for this, without getting too involved for OS X. First is install 4.13.0 preview 1. Any version < than 4.12.5 will keep coming up with this error (except for 4.6 or 4.7, but that’s so far back!).
The second is very drastic. I created a separate account on my Mac, and logged in as it, then removed the Users/Shared/UnrealEngine/4.12 directory, went in as Epic Launcher, then reinstalled the 4.12.5 Engine.
Worked like a charm!
The final fix, which I confirmed worked after I did the above “new account” was deleting
/Users/username/Library/Application Support/Epic/UnrealEngine/4.12/DerivedDataCache/Boot.ddc.
Wish I would’ve found that first.