Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires vimeo/psalm ^5.13 -> satisfiable by vimeo/psalm[5.13.0, ..., 5.x-dev]. - infection/infection[0.29.0, ..., 0.29.x-dev] 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 -> satisfiable by infection/infection[0.29.0, ..., 0.29.x-dev].