Hello FedeKoro,
It sounds like something may have happened to your files during download causing them to be corrupt. If you are able please try and delete the files that you have downloaded and uninstall any files that you may have installed on the first attempt. After that download the files from unrealengine.com again and re-install. Hopefully this will fix the issue. I am including a link to a webpage that provides help with launcher related issues.
If the re-install does not work and you have tried all of the suggested methods on the webpage, just let me know and we will look into your issue further.
Thanks,