BAD NEWS:

After learning how to create a NavMesh (an enemy AI), I added it to the game, with only one line of code I was able to make the zombies move to a specific coordinate.

After that I wanted to create a player controller, in the beginning, I thought a first-person controller could work for my game, but no. A third-person controller would be better because it allows the player to look behind him, soo he could see if zombies are attacking him from behind.

But a third-person controller is difficult to create, soo I decided to download one from the asset store, but all the assets I found are only compatible with version 2020 of Unity and I was using version 2019.4.

So I switched to version 2020 and I was surprised with 9 error messages in the console that are impossible to fix:

The errors were coming from the UnityEngine, not from my Scripts, so I started the project from the beginning, luckily I was able to copy-paste my files from the corrupted project, and the good thing is that "having this problem now means I won't have it later :)".