Incorrect branch when adding modules with composer
Make sure you are using the correct repository endpoint for Composer. For Drupal 8, it is https://packages.drupal.org/8.

Your composer.json file should contain a repository key like so:

{ 
    "repositories": { 
        "drupal": {
            "type": "composer",
            "url": "https://packages.drupal.org/8" 
        }
    }
}