Why isn't my composer installation working?
As shown in the example reported in the documentation page you linked, you need to add drupal/ to the machine name of the module you want to install. The given example is exactly for the Devel module.

composer require drupal/devel:~1.0


The name of the packages handled from Packagist has the format vendor/package, so if you simply use the machine name of the module, Composer will not be able to find the package you meant.

Remember to run the commands from the directory containing the composer.json and the composer.lock files, not from the core directory.