Hello! : )
Maybe it’s a download issue. GitHub isn’t so friendly with unreal projects yet. : S
This button doesn’t work in this context, this will download all assets as “corrupted” files with only 1kb size and the project will not work:

The same will occur if you’re cloning the repository without Git LFS installed.
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If you’re cloning the repository, ensure that you have Git LFS installed and call this command inside the project’s folder after the clone:
git submodule update --init --recursive -
If you’re downloading the .zip, use this link: Releases · lucoiso/UEProject_Elementus (github.com)
About the C#: Unreal use it for the Build Tool system. To build this project you’ll only need Visual Studio w/ Game development with C++ module.
Note: The engine also requires .NET but is not specific of this project.
If the problem still persists, let me know!