Your requirements could not be resolved to an installable set of packages. Problem 1 - guysartorelli/silverstripe-composable-validators is present at version 2.1.x-dev and cannot be modified by Composer - silverstripe/recipe-cms 5.1.x-dev requires silverstripe/session-manager 2.1.x-dev -> satisfiable by silverstripe/session-manager[2.1.x-dev]. - silverstripe/session-manager 2.1.x-dev requires silverstripe/framework ^5.1 -> satisfiable by silverstripe/framework[5.1.0-beta1, 5.1.0-rc1, 5.1.x-dev, 5.x-dev]. - silverstripe/installer 5.1.x-dev requires silverstripe/recipe-cms 5.1.x-dev -> satisfiable by silverstripe/recipe-cms[5.1.x-dev]. - guysartorelli/silverstripe-composable-validators 2.1.x-dev conflicts with silverstripe/framework 5.1.0-rc1. - guysartorelli/silverstripe-composable-validators 2.1.x-dev conflicts with silverstripe/framework 5.1.x-dev. - Root composer.json requires silverstripe/installer 5.1.x-dev -> satisfiable by silverstripe/installer[5.1.x-dev].