Preferences:
/Users/[username]/Library/Preferences/com.adobe.Lightroom6.plist
In Mac OS 10.9.x and later, to reset Lightroom's preferences, after you delete the above file, restart your Macintosh. Also, beginning with Mac OS 10.9, to fully reset your Lightroom preferences, in addition to deleting the plist file, press the shift + option keys immediately after you restart Lightroom, and select Reset Preferences from the resulting dialog box.