Problème installation Symfony 4

Bonsoir quand j'essai d'installer symfony 4 avec composer symfony create-project symfony/website-skeleton MaSuperAgence ou avec packagist.org composer symfony/symfony il me donne cette erreur : Using version ^4.2 for symfony/symfony ./composer.json has been updated Loading composer repositories with package information Updating dependencies (including require-dev) Your requirements could not be resolved to an installable set of packages. Problem 1 - Conclusion: don't install symfony/symfony v4.2.7 - Conclusion: don't install symfony/symfony v4.2.6 - Conclusion: don't install symfony/symfony v4.2.5 - Conclusion: don't install symfony/symfony v4.2.4 - Conclusion: don't install symfony/symfony v4.2.3 - Conclusion: don't install symfony/symfony v4.2.2 - Conclusion: don't install symfony/symfony v4.2.1 - don't install symfony/yaml v3.4.15|don't install symfony/symf