Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires phpunit/phpunit ^12.0.0 -> satisfiable by phpunit/phpunit[12.0.0, 12.0.1]. - Root composer.json requires psalm/plugin-phpunit ^0.19.2 -> satisfiable by psalm/plugin-phpunit[0.19.2]. - phpunit/phpunit[12.0.0, ..., 12.0.1] require sebastian/diff ^7.0.0 -> satisfiable by sebastian/diff[7.0.0]. - psalm/plugin-phpunit 0.19.2 requires vimeo/psalm dev-master || ^6 -> satisfiable by vimeo/psalm[6.0.0, ..., 6.4.0]. - sebastian/diff[4.0.0, ..., 4.0.1] require php ^7.3 -> your php version (8.3.16) does not satisfy that requirement. - vimeo/psalm[6.0.0, ..., 6.4.0] require sebastian/diff ^4.0 || ^5.0 || ^6.0 -> satisfiable by sebastian/diff[4.0.0, ..., 4.0.6, 5.0.0, ..., 5.1.1, 6.0.0, 6.0.1, 6.0.2]. - You can only install one version of a package, so only one of these can be installed: sebastian/diff[4.0.0, ..., 4.0.6, 5.0.0, ..., 5.1.1, 6.0.0, 6.0.1, 6.0.2, 7.0.0]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.