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).