Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires vimeo/psalm 5.26.1 -> satisfiable by vimeo/psalm[5.26.1]. - vimeo/psalm 5.26.1 requires php ^7.4 || ~8.0.0 || ~8.1.0 || ~8.2.0 || ~8.3.0 -> your php version (8.4.0-dev) does not satisfy that requirement. Problem 2 - 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 -> satisfiable by vimeo/psalm[5.26.1]. - vimeo/psalm 5.26.1 requires php ^7.4 || ~8.0.0 || ~8.1.0 || ~8.2.0 || ~8.3.0 -> your php version (8.4.0-dev) does not satisfy that requirement.