Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires mantle-framework/testkit ^0.11 -> satisfiable by mantle-framework/testkit[v0.11.0, ..., 0.11.x-dev]. - mantle-framework/testkit[v0.11.0, ..., 0.11.x-dev] require mantle-framework/support ^0.11 -> found mantle-framework/support[dev-main, v0.11.0, ..., 0.11.x-dev (alias of dev-main)] but it conflicts with your root composer.json require (^0.12).