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, 0.20.1]. - Root composer.json requires vimeo/psalm ^5.13 -> satisfiable by vimeo/psalm[5.13.0, ..., 5.26.1]. - psalm/plugin-phpunit[0.20.0, ..., 0.20.1] require 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.