How to reset the built-in Dictionary app for Look Up function?
This is a bug specific to MacBook Pro 16-inch with Catalina 10.15.X (In my case X=2), so reinstall the OS will not help. If you're in the same situation, I can solve this problem for you as follows, which is my conclusion from the Apple Communities thread discussing the same problem:


Remove all folders under /System/Library/AssetsV2/PreinstalledAssetsV2/InstallWithOs/com_apple_MobileAsset_DictionaryServices_dictionaryOSX.
Restart your computer. (In my case the Look Up didn't show some dictionary definition before I restarted my computer.)