Error opening the project

I cloned the files with the Git Bash and all that things, but, when I open the project with Unity 2018.3.8 it says it didn´t find a lot of files.

 

Some of us have been submitting patches for review (pull request) to fix this issue and others. I suspect this github repo is abandoned by the devs, but you can still look at the file diffs from the pull requests here:
https://github.com/Unity-Technologies/FPSSample/pulls
You want to click through "Remove https://staging-packages.unity.com registry in manifest.json" and look at the file diff. Don't overlook the removal of the comma.