Your requirements could not be resolved to an installable set of packages.
Problem 1
- silverstripe/framework[4.11.16, ..., 4.11.x-dev] require guzzlehttp/guzzle ^7 -> found guzzlehttp/guzzle[7.0.0-beta.1, ..., 7.8.x-dev] but it conflicts with your root composer.json require (^6.3).
- silverstripe/recipe-cms 4.11.x-dev requires silverstripe/recipe-core 4.11.x-dev -> satisfiable by silverstripe/recipe-core[4.11.x-dev].
- silverstripe/installer 4.11.x-dev requires silverstripe/recipe-cms 4.11.x-dev -> satisfiable by silverstripe/recipe-cms[4.11.x-dev].
- silverstripe/recipe-core 4.11.x-dev requires silverstripe/framework 4.11.x-dev -> satisfiable by silverstripe/framework[4.11.x-dev].
- Root composer.json requires silverstripe/installer 4.11.x-dev -> satisfiable by silverstripe/installer[4.11.x-dev].