Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires psalm/plugin-phpunit ^0.19.2 -> satisfiable by psalm/plugin-phpunit[0.19.2]. - psalm/plugin-phpunit 0.19.2 requires vimeo/psalm dev-master || ^6 -> found vimeo/psalm[6.0.0, ..., 6.4.0] but it conflicts with your root composer.json require (^5.26.1). Problem 2 - Root composer.json requires infection/infection ^0.29.10 -> satisfiable by infection/infection[0.29.10]. - Root composer.json requires vimeo/psalm ^5.26.1 -> satisfiable by vimeo/psalm[5.26.1]. - infection/infection 0.29.10 requires nikic/php-parser ^5.3 -> satisfiable by nikic/php-parser[v5.3.0, v5.3.1, v5.4.0]. - vimeo/psalm 5.26.1 conflicts with nikic/php-parser v4.17.0. - vimeo/psalm 5.26.1 requires nikic/php-parser ^4.17 -> satisfiable by nikic/php-parser[v4.17.0, ..., v4.19.4]. - You can only install one version of a package, so only one of these can be installed: nikic/php-parser[v4.17.0, ..., v4.19.4, v5.0.0, ..., v5.4.0]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.