The homepage value in the package.json is incorrect as it is not a valid URL (it is missing the protocol e.g. https://). This key is not required for most React projects as I outlined in the following post
I suggest removing the homepage key.
The path for the favicon.ico does not work when building. Any asset not referenced inside React e.g.
import Carpeta from '../../assets/icons-languages/icon-animated/Carpeta.gif'
does not get copied to the output directory during build. The favicon.ico should go into the public directory alongside the index.html and /src/assets removed from the href inside index.html.
I can confirm this works as I have tested your repository locally.
Do note none of this is anything to do with Netlify. It is entirely the setup of the project. You can test this locally before deploying to Netlify by running npm run build and checking the output. Using a package such as serve or http-server to serve the locally built content and view as you would when deployed is a good idea too.