If you already have the BP_MasterTurret blueprint in your project then you do not need to follow this guide.
Close your project and go to the project files in windows file browser. Then go to this folder.
Game/SmartAI/Blueprint/AI/Characters/
and paste in the file linked below.
https://www.dropbox.com/s/p9yaq7w6b5fwk24/1.0.12%20Missing%20BP_MasterTurret%20file.zip?dl=0