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]. - psalm/plugin-phpunit 0.19.0 requires vimeo/psalm dev-master || ^5@beta || ^5.0 -> found vimeo/psalm[5.0.0, ..., 5.23.1] but it conflicts with your root composer.json require (^4.5). Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.