I see a couple issues in your log. I believe the real failure is this error:
ERROR: Invalid SDK MacOSX.sdk, not found in /Library/Developer/CommandLineTools/Platforms/MacOSX.platform/Developer/SDKs
I’m not a Mac guy, but some quick googling suggests you can fix this by installing XCode.
I also see your steam dlls are not loading correctly
LogOnline:Display: STEAM: Loading Steam SDK 1.32
LogMac:Warning: dlopen failed: dlopen(/Users/Shared/Epic Games/UE_4.13/Engine/Binaries/Mac/UE4Editor.app /Contents/MacOS/libsteam_api.dylib, 5): image not found
LogOnline:Warning: STEAM: Steam SDK 1.32 libraries not present at or failed to load!
That error suggests the files are just missing.