Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires react/filesystem 0.1.2@dev -> satisfiable by react/filesystem[v0.1.2]. - Root composer.json requires wyrihaximus/async-test-utilities ^8.0.5 -> satisfiable by wyrihaximus/async-test-utilities[8.0.5]. - react/filesystem v0.1.2 requires react/promise ~2.2 -> satisfiable by react/promise[v2.2.0, ..., 2.x-dev]. - wyrihaximus/async-test-utilities 8.0.5 requires react/promise ^3.2.0 -> satisfiable by react/promise[v3.2.0, v3.3.0, 3.x-dev]. - You can only install one version of a package, so only one of these can be installed: react/promise[v1.1.0, ..., 1.x-dev, v2.0.0, ..., 2.x-dev, v3.0.0, ..., 3.x-dev]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.