git clone https://github.com/Homebrew/homebrew-cask-cask /usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask-cask
Password for ‘https://email@example.com’:
remote: Invalid username or password.
fatal: Authentication failed for ‘https://github.com/Homebrew/homebrew-cask-cask/’
It looks as though the repository you’re trying to clone either doesn’t exist or is private. Either way, you don’t seem to have access to it.
If you’re using a tutorial, it could be that it’s out of date and you need to find something newer.