Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires psalm/plugin-phpunit ^0.19.0 -> satisfiable by psalm/plugin-phpunit[0.19.0]. - infection/infection[0.29.0, ..., 0.29.6] require infection/mutator ^0.4 -> satisfiable by infection/mutator[0.4.0]. - Conclusion: don't install infection/mutator 0.4.0 (conflict analysis result) - Root composer.json requires infection/infection ^0.29.0 -> satisfiable by infection/infection[0.29.0, ..., 0.29.6]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.