By default, the developer console is locked in the standard retail version of The Witcher 3 . You must enable it via the game files before you can input commands. For PC Users (Steam / GOG / Epic Games) Close The Witcher 3 completely.
If a quest item is trapped inside a chest that refuses to show an interaction prompt, point your crosshair at it and open the console: chtUnlock() or Unlock() witcher 3 complete quest console command