Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires psalm/plugin-phpunit ^0.18.4 -> satisfiable by psalm/plugin-phpunit[0.18.4]. - psalm/plugin-phpunit 0.18.4 requires vimeo/psalm dev-master || dev-4.x || ^4.7.1 || ^5@beta || ^5.0 -> found vimeo/psalm[4.7.1, ..., 4.30.0, 5.0.0, ..., 5.26.1] but it conflicts with your root composer.json require (^6.0.0). Problem 2 - Root composer.json requires vimeo/psalm ^6.0.0 -> satisfiable by vimeo/psalm[6.0.0]. - vimeo/psalm 6.0.0 requires nikic/php-parser ^5.0.0 -> found nikic/php-parser[v5.0.0, ..., v5.4.0] but it conflicts with your root composer.json require (^4.16.0). Problem 3 - roave/infection-static-analysis-plugin 1.36.0 requires infection/infection 0.29.10 -> satisfiable by infection/infection[0.29.10]. - roave/infection-static-analysis-plugin 1.32.0 requires vimeo/psalm ^4.30.0 || ^5.0.0 -> found vimeo/psalm[4.30.0, 5.0.0, ..., 5.26.1] but it conflicts with your root composer.json require (^6.0.0). - roave/infection-static-analysis-plugin[1.33.0, ..., 1.35.0] require vimeo/psalm ^4.30.0 || ^5.15 -> found vimeo/psalm[4.30.0, 5.15.0, ..., 5.26.1] but it conflicts with your root composer.json require (^6.0.0). - infection/infection 0.29.10 requires nikic/php-parser ^5.3 -> found nikic/php-parser[v5.3.0, v5.3.1, v5.4.0] but it conflicts with your root composer.json require (^4.16.0). - Root composer.json requires roave/infection-static-analysis-plugin ^1.32.0 -> satisfiable by roave/infection-static-analysis-plugin[1.32.0, ..., 1.36.0]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.