Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires react-parallel/pool-tests ^5.1.0 -> satisfiable by react-parallel/pool-tests[5.1.0, 5.2.0]. - react-parallel/pool-tests 5.1.0 requires wyrihaximus/async-test-utilities ^10.1.0 -> found wyrihaximus/async-test-utilities[10.1.0, ..., 10.7.0] but it conflicts with your root composer.json require (^12.1.0). - react-parallel/pool-tests 5.2.0 requires wyrihaximus/async-test-utilities ^10.4.0 -> found wyrihaximus/async-test-utilities[10.4.0, 10.5.0, 10.6.0, 10.7.0] but it conflicts with your root composer.json require (^12.1.0). Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.