Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires silverstripe/installer 5.1.0-beta1, found silverstripe/installer[dev-4.8-beta2, dev-post-2.4, 2.4.9, ..., 2.5.x-dev (alias of dev-post-2.4), 3.0.2.1, ..., 3.x-dev, 4.0.0-alpha1, ..., 4.x-dev, 5.0.0-alpha1, ..., 5.x-dev] but it does not match the constraint.