@SY Yoon:
Can you post the output of evaluating $Version
?
Also, can you try disabling the suggestions bar (it's in Preferences -> Interface) and restarting Mathematica? In older versions there was a bug where the suggestions bar somehow caused some symbols to become undefined. I assume it's much more robust in recent versions, but it's worth a try.
BTW it's not a macOS issue. It works perfectly fine for me on macOS.