i make a widget and then add to viewport but how can I tell it after all turrets been destroyed
You have some kind of game controller that keeps track of the game state, right? This might be the game mode, or the game instance, in most multiplayer games, or it could just be the player controller in a single-player game.
So, this object needs to know which are all the objects to be destroyed for this goal in the game. Each destroyable object then needs to call into this object to say “I’ve been destroyed” when they get destroyed, and the object can keep track of this. When the list of destroyed objects is the same as the list of objects to be destroyed, you win!
There are several variations you could use, too. You could for example tag everything that needs to be destroyed with some tag, and then iterate over all objects with that tag, checking whether they are destroyed. (You could find all objects with this tag in EventBeginPlay in your game managing objects, for example.)