"Blueprint could not be loaded because it derives from an invalid class. Check to make sure the parent class for this blueprint hasn't been removed!"

Okay so.
I wanted to make a game but unfortunately, I had no idea for the weapons.
Now I go ahead and watch this video on Youtube: The Arena | Competitive Multiplayer - YouTube
And I loved the weapons. Then I realized that they uploaded the project files on Github. So I cloned the files to add it in my game.
Here’s the files: https://github.com/Jeff-Stapleton/TheArena

Now I clone the project and I open the uproject files and then nothing happened. Just black screen. The content and the blueprint is all fine it loads well.
But I can’t even do anything cause everything is black.

So I made a new C++ project (Even though I don’t know C++)
and then I extracted the content from the other project to this one.
And now when I load these it says: “Blueprint could not be loaded because it derives from an invalid class. Check to make sure the parent class for this blueprint hasn’t been removed!”

Also, This is for any blueprint class I open. None of them work! But they load when I open The Arena project.
But when I hover over the class it says “Parent class: Arena.(something)” And then I open it, The error comes. Then I hover again it says “none”

Where is the parent class?

If you renamed the project you need to change all references to TheArena.h in the c++ source.