I don’t know how to explain this based on the info you provided so far 
If the Engine/Shaders folder exists, I don’t know why the check for its existence would fail.
Try this, please:
From Finder’s menu bar select Go->Go to Folder and enter
~/Library/Logs/Unreal Engine/Editor
This should open a window with log files from your attempts to run the editor. The newest one should simply be called UE4.log. Please attach it to this thread for me to take a look at.