I am not doubting your experience, I am simply trying to understand what it could possibly mean 
Packaging Just Fine
I can package just fine for shipping builds in my test project.
The key line is this:
/Game/ThirdPersonCPP/ThirdPersonOverview.ThirdPersonOverview_C' was not found!
**Issue Identified**
Ahh I think I see what is going on here, in my sample project that I distribute with this plugin, I have a test map called:
ThirdPersonOverview
When you extracted the plugin to your project, somehow there's something that is looking for that map, there's a stray reference somewhere somehow, that is still looking for that map.
**Perhaps you pulled in other asset content from my sample project, but not the map itself?
There wouldn't be a reference in the code, it must be some content you ported over**
You can test my theory by pulling into your project the map from my sample project as follows:
```
Content/ThirdPersonCPP/ThirdPersonOverview
```
If you do that you should be able to package.
Directory structure has to be exact as it is a string asset reference
**The Solution **
After you package once successfully with my test map in your project in the exact correct location, try deleting the map from your project, and then right-click on your project root and say Fix Up All Redirectors in Folder
Then try packaging again, and hopefully all will work from that point onward.