Clearly the dark mode is the modern one! Jokes aside, I just realized that there THREE menu options on that toolbar: hamburger, kebab, and waffle! I realize they do different things, but no wonder people are confused by and scared of computers. Also, now I'm hungry!

Are you trying the terminal commands with sudo? You could also try logging in as root user with the password you used during setup.

This is a really good tutorial playlist starting with very basics, which you could skip/speed past based on your experience level.