Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires psalm/plugin-phpunit ^0.20.0 -> satisfiable by psalm/plugin-phpunit[0.20.0]. - Root composer.json requires vimeo/psalm ^5.16.0 -> satisfiable by vimeo/psalm[5.16.0, ..., 5.26.1]. - psalm/plugin-phpunit 0.20.0 requires psalm/psalm-plugin-api ^0.1 -> satisfiable by psalm/psalm-plugin-api[0.1.0]. - psalm/psalm-plugin-api 0.1.0 conflicts with vimeo/psalm <7.0.0. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.