Composer download Drush 9 - But conflict errors
When you request composer require drush/drush:9.*, you are telling Composer that you want the latest stable release of Drush on the 9.x branch.  However, Composer considers dev, alpha, beta and even RC releases to be unstable.  These releases can be downloaded in one of two two ways:


Create a composer.json file, manually add the release designation you want (e.g. 9.*), set the minimum stability to dev or alpha, or some other applicable value, and then run composer install (or composer update, if you already have a composer.lock file).
Explicitly request the version you want -- e.g. composer require drush/drush:9.0.0-alpha1, or ``composer require drush/drush:dev-master`.