The issue is Apple decided to entirely remove support for running any 32 bit executables. Older 64 bit versions of the Unity Editor still had some functionality handled by 32 bit code. That all broke with what Apple decided to do.
The latest version of Unity 2017.4.x LTS has the fix backported, and is the oldest version of Unity getting it. Unity 5 has been out of support for too long and will not be getting the backport. You’ll have to either use a Windows machine or an older OS X version which still has 32 bit support in order to test with Unity 5.
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