Your requirements could not be resolved to an installable set of packages.
Problem 1
- silverstripe/recipe-cms 5.1.x-dev requires silverstripe/cms 5.1.x-dev -> satisfiable by silverstripe/cms[5.1.x-dev] from composer repo (https://repo.packagist.org) but silverstripe/cms is the root package and cannot be modified. See https://getcomposer.org/dep-on-root for details and assistance.
- silverstripe/installer 5.1.x-dev requires silverstripe/recipe-cms 5.1.x-dev -> satisfiable by silverstripe/recipe-cms[5.1.x-dev].
- Root composer.json requires silverstripe/installer 5.1.x-dev -> satisfiable by silverstripe/installer[5.1.x-dev].