Error: unable to locate asset entry in pubspec.yaml: "assets/fonts/Lato-Regular.ttf"

The assets folder should be in the root path (/). If you want to keep it in the lib folder, put lib in front of assets like this:

//new path
lib/assets/fonts/Lato-Regular.ttf

Sidenote: To avoid having to import fonts manually, I recommend the google_fonts package. It has hundreds of fonts including Lato and you can access it with GoogleFonts.lato() to get a default TextStyle with the Lato font without having to import it via the pubspec.yml file.