I’m going to suggest you do what all of us have to do at some point - make a new project and walk through the setup process from new project to packaged build (say, of the third person BP). Then go back to your game and go through each of the steps again.
If that fails, try moving your config folder, letting it make a new one, setting up the build stuff again and trying again. If that fixes it then your problem is a config setting. I had to do this recently as well.